Truck started to sputter

Are you pulling on the return spring or are you opening the throttle against the pull of the return spring? Tugging on the spring won't do anything. Besides, if something broke such as the throttle cable, the engine speed would just go back down to idle like normal; it wouldn't sputter and stall.

More likely you have a fuel supply problem. A fast way to prove that is to pour a little fuel down the middle of the carburetor. Use the equivalent of about 1/8" in a small soup can. Push the gas pedal a little so enough air can get in, then crank the engine. It will run fast for a few seconds, then die.

Another check is to hold the choke plate open if the engine is cold, then look down the middle of the carburetor while you push the throttle open by hand. Each time you do that you should see one or two streams of gas spraying in, depending on which engine you have and whether it's a one, two, or four barrel carburetor. If there's no squirts of fuel, the float bowl is empty. The previous test will confirm that. It's not common to have a mechanical problem with the fuel pump or steel fuel lines without also have an external leak. The pickup screen in the tank could be plugged too but the symptoms will act a little differently than what you described.

Dump a little gas in to be sure the engine runs. If it does not, the nylon teeth on the camshaft sprocket may have stripped off so the camshaft is not turning. The camshaft drives the fuel pump but it also drives the distributor. If you also have no spark, that gear would be a good suspect. In case you DO have spark, when you test for it, stick a screwdriver in the end of a spark plug wire but be sure to hold it close to the engine so the spark can jump the gap. You have an HEI system with the really big distributor cap, and that spark IS going to go somewhere. If you hold the screwdriver too far away for the spark to jump, it will find another path to ground and that usually leads to more things to fix later.

If you have no spark either along with no fuel, remove the distributor cap and watch to see if the rotor turns while cranking the engine. If it doesn't, head for the timing chain.

With all that said, I suspect you simply have a fuel problem, and here's why. If anything would have happened to the timing chain or sprocket, the engine would have misfired, backfired, and died right away. Up to that point the fuel pump would have been working so the float bowl would have been full right when the engine died. You would still be getting nice streams of fuel when you worked the throttle by hand and watched in the carburetor. There's enough gas in the float bowl for the engine to idle for about two minutes or run on the highway for about half a mile.

The next thing is to check the gas supply system. Since you just bought gas, we know the tank isn't empty, but if it had just run out as you pulled into the station, there would be air in the steel line going to the fuel pump. Some pumps are not good at drawing a prime from the tank, but they work just fine when filled with liquid. I have one car that did that since it was new. I regularly ran it out on the highway, then dumped in a measured gallon from a can to see exactly what I was getting for mileage, but to get it to fire, I had to dump some down the carburetor. The higher speed of the running engine is what that pump needed to suck fuel from the tank. This is where you might want to dump some down your carburetor to verify everything else is working.

We can discuss further the next things to check, but most of the common stuff will not cause a sudden stalling as you described. The things I'm thinking of would have caused some other intermittent running problems before now.

Sorry for the delay. Tornado went through last night and power went out. 'Puters don't work well without power!

So we know spark and timing are still okay.

First thing to do is inspect the steel fuel line that runs along the frame rail to see if there's any wet spots or any stains that indicate were it has been leaking. Stains will usually have dirt on them too that stuck to the gas. If there is a leak, the pump will suck up air instead of gas. That too usually won't occur all of a sudden without warning but it's not something to overlook.

Next step is to disconnect the fuel line going into the pump, then either connect a vacuum gauge to see if it can pull a little vacuum, or you can connect a short piece of line and place the other end in a can of gas to see if it can draw any up. If you try the can of gas, you might want to dump some more down the carburetor. When the engine runs normally for a few seconds, that will help the pump draw up gas from the can. If you just crank the engine, it will take some time to draw the gas up, then it needs to pump enough to fill the float bowl before the engine will start and run. That could take half a minute of cranking, or more. Also be aware whatever you connect to the pump must be leak-free. Air can sneak past the threads and get sucked in instead of pulling up fuel.

If you get a small vacuum reading, or if it pumps fuel from a can, there is something wrong with the pickup assembly in the fuel tank. Most common was a plugged screen on the end of the pipe. If there is no vacuum reading or no fuel being sucked up, the fuel pump is bad or there's a problem with what runs it. Most GM engines have an eccentric lobe on the camshaft to run the pump, and that isn't going to wear away suddenly. Most of their engines also use a push rod from that lobe to the lever on the pump. I've never heard of that causing a problem. That would just leave the pump itself. Lucky you, mechanical fuel pumps are real inexpensive. That push rod makes them a little miserable to replace on GM engines but not nearly as much as the newer electric pumps inside the gas tanks.