Cannot go over 35 mph

From your description of the symptoms, this sounds like what I've run into four times, twice with carbureted engines and twice with a fuel injected engine. The culprit was a plugged or collapsing filter screen on the fuel pick-up tube in the gas tank. With carbureted engines, the problem gets worse as you try to accelerate. Fuel volume can't keep up. This is a lot more elusive with fuel injection. The problem occurs when the largest volume of fuel is being pumped, which is during coasting from highway speed.

The second time this happened to my '88 Grand Caravan, it only occurred when I was pulling a huge tandem-axle enclosed trailer that was bigger than the van. I ran it for a year and a half with a fuel pressure gauge tied to the radio antenna. The fuel pressure would slowly drop from 50 psi to 20 psi over about half a minute, then the engine would start to sputter at 15 psi. I figured out that if I lifted the accelerator pedal for a fraction of a second, the pressure would pop back up, then it would run fine for another half mile. I nursed it 55 miles that way.

Since you have a carburetor, watching fuel pressure might not work since normal pressure is so low. If you have a gauge meant for use with carburetors, that's the place to start. If you have two gas tanks, one clue is the engine will run better on the other tank.

Another potential clue is every time this happened to me, the fuel volume was fine for at least the first 15 minutes, then, once the problem started to occur, I had to sit on the side of the road for about five minutes, then I could make it another couple of miles before it occurred again.

Also look for dry-rotted rubber fuel hoses where air could be getting sucked in.

Fuel pumps in the tank are not common with carburetors, so I wasn't sure why you had two of them. The only time I've run into that was on the Chevy Chevettes. Also be aware you can buy most in-tank fuel pumps two ways. Most people go for the less-expensive pump and motor only that gets transplanted into the housing. The screen gets reused / overlooked. When the entire housing is available with the pump already in it, that comes with a new screen, but it's usually still a lot less expensive to buy just the pump and motor, then buy a new screen separately.

If you think this isn't fuel-related, (keep in mind revving the engine in neutral requires very little gas compared to hard acceleration on the road), the next suspect might be a tight universal joint. I ran into this when my boss was looking at buying a used Ford van for his TV repair shop. I know this sounds strange, but we couldn't get the van to go faster than 55 mph with it floored. Finally at the end of one test-drive, we heard a little squeaking from the rear u-joint that gave it away. The dealer had both u-joints replaced, then acceleration was back to normal. Since then I've run into that a few more times. The only thing different is there was no sputtering.

Consider watching what happens to ignition timing when you raise engine speed. If it advances in steps, but keeps advancing, the advance weights in the distributor are sticking. If timing bounces back and forth, especially while holding engine speed mid-range and steady, suspect a worn bushing in the distributor or a stretched timing chain. Normally these two things cause surging, but you can still reach highway speed.

Another weird one had to do with a dry-rotted rubber hose inside the tank on Ford Tempos. I never had one apart so you'll have to visualize this. The symptom was the engine would run fine as long as the tank was at least half-full of gas. When the level dropped below that, air would intermittently get sucked in. The symptom was it acted like it was running out of gas.

Had an old late '60s Pontiac wagon that ran fine around town, but stalled when trying to accelerate to highway speed. Ended up being the hood silencer pad was falling down and blocking the inlet on the air filter housing.

Ford had horrendous problems with their front brake calipers. Their design and how they're held to the mounting knuckle is asking for trouble with sticking brakes. I worked at a Sears Auto Center in the '80s. We rebuilt the calipers and wheel cylinders as a part of every brake job. We even had an almost-new Ford van come in from out-of-state with smoking front brakes at 7,000 miles. The local Ford dealer told the fellow there was nothing they could find wrong, and warranty doesn't cover brakes. We also had a half dozen Ford vans for the service department. We were replacing brakes on those all the time. Before you go looking for the hard stuff, check for badly-dragging brakes and rule that out as a possible cause. Stop on a slight incline, shift to "neutral", then see if it creeps down-hill on its own.

The point is, ... This is very unlikely to be a spark-related problem. You need spark, fuel, compression, and correct timing of all the events. Compression isn't going to be intermittent or speed-sensitive. Ignition timing will cause reduced power and fuel mileage, but it won't limit maximum speed this badly. I think you would have noticed problems with the brakes, so that leaves fuel. Check if the secondaries are opening up on the carburetor. You don't need them to raise engine speed in "neutral", but you do to get up to highway speed. If the secondaries aren't opening up, that's still a fuel problem, but not a problem with the fuel supply system or fuel pressure.

You don't think this is fuel-related, so I'm looking for other possibilities. Dragging disc brakes is fairly common so I wanted to mention it and not overlook anything that might give you an idea of what's causing this.

There were two ignition systems used by Ford in '84. One used a TFI module, (thick film integrated), bolted to the side of the distributor. That's shown at the top of the photo. More common for '84 was the ignition module shown at the bottom of the photo. That is not a computer as we think of an electronic control module, (ECM) or power train control module, (PCM). Those have memories and can set diagnostic fault codes to indicate a circuit that needs further diagnosis. The modules in this photo simply take the place of the breaker points in older systems. They drive the ignition coil, to create a spark when they're triggered by a pulse from the pick-up coil inside the distributor. That pick-up coil sits where the breaker points used to be.

When you have a true Engine Computer, (ECM or PCM), those calculate ignition timing which you can't change or adjust. The pulses from the pick-up assembly start out seriously advanced, often as much as 30 - 40 degrees, then the computer calculates how much delay is needed to fire the ignition coil at just the right time. It looks at coolant temperature, intake air temperature, engine load, (vacuum), engine speed, and on all except Chrysler engines, the weight of the incoming air, (mass air flow sensor), to calculate fuel needs and ignition timing. Newer systems no longer use a distributor, but the clue when you do not have an Engine Computer is the vacuum advance unit on the side of the distributor. That, and the internal advance weights won't be there when there's a computer to adjust the amount of timing advance. The first true computer was used on '77 Chryslers but it only adjusted ignition timing and it couldn't set diagnostic fault codes. Those systems were primitive by today's standards so the entire system could be diagnosed in a few minutes. Today we wouldn't have any idea where to start without fault codes, and we could waste weeks trying to solve some problems without them. Starting in '78, the Engine Computers on some Chrysler models also adjusted fuel / air mixture, but they still used carburetors.

As far as ignition module failures, they can be intermittent, but intermittent stalling is much more likely to be caused by the pick-up coil inside the distributor. That can be caused by the constant flexing of the wires as the vacuum advance unit moves it back and forth, or it can be caused by one of the ends of the coil of wire breaking off its terminal. The real common symptom when the wire breaks at the terminal is the engine starts fine cold and runs okay as long as you keep moving. Natural air flow keeps the distributor cool. It's when you stop for a few minutes with a hot engine, such as when stopping for gas, that you'll have a crank / no-start. It's caused by "hot soak" when engine heat migrates up to the distributor and causes the pick-up coil to expand. That lets the break in the wire open up. The common observation is the engine will start and run fine after cooling down for about an hour or two.

Each manufacturer had their common failures with these early systems, but none of them caused the symptom you described. Since you mentioned the engine sputters at 35 mph, perhaps this is when the vacuum advance is just kicking in, and the wires to the pick-up coil are flexing and breaking. A test for this would be to disconnect the vacuum hose at the distributor, and plug it, then go for a drive and see what happens. You'll still have mechanical advance that's needed at higher engine speeds, but that doesn't move the pick-up coil or flex its wires. If there's no sputtering and you can reach highway speed, we'll discuss how to test the pick-up coil, if you want to, to verify its failure.

Please keep us updated. I'm anxious to know what is causing this so I can add it my memory banks.