Truck still getting hot. Is motor shot?

Those volumes are only correct when you're starting out with a totally empty system which you are not. Another example: most engines in the '70s took 5 quarts of oil at an oil change but they take 7 quarts when you're filling a freshly rebuilt engine. Two quarts is stuck in the many passages, lifters, rocker arm shafts, etc. Same is true of your cooling system. Removing the radiator still leaves coolant in the lower half of the block, heater core and hoses, etc.

The best you can do is fill with a 50/50 mix, run it to mix it up, then check the freeze point and add just water or antifreeze in the reservoir to fine tune it. Minus 45 to 50 degrees is the best you can get. If the concentration of antifreeze is too high, the freeze point starts to go back up, but most testers lie beyond that point and give false readings because they are measuring the weight of the coolant compared to that of straight water. Engines will often overheat too if there is too much antifreeze in the coolant. It can't hold as many BTUs of heat as water, so it is very inefficient at moving that heat to the radiator.

If you're still having overheating problems, look for things like a missing shroud around the radiator, a weak fan clutch, a missing rubber strip under the front edge of the hood, or anything missing on the core support that lets air bypass the radiator. A 440 probably calls for a three-core radiator. Check for corroded and crumbling cooling fins if it was a used one.

The test for that involves what I call the "sniffer" test. Air from the radiator is drawn through a glass cylinder with two chambers partially filled with a special dark blue liquid. If combustion gases are getting into the cooling system, the liquid will turn bright yellow.

The test you're thinking of is for finding the source of an external leak. You add a bottle of dye, drive a while, then search with a black light. The dye will show up as a bright yellow stain.

When the overheating occurs, feel the two radiator hoses and run your hand across the radiator from side to side about half way down. If they are cool, the hot coolant isn't getting to the radiator. If the lower hose is real hot, the heat isn't being given off by the radiator. That happens when it is old and corroded. The paper thin fins will crumble from the slightest touch. That tends to make the engine run warmer than normal at highway speeds. Air flow / fan problems generally do not cause problems at higher speeds because there is natural air flow. Another clue to the ineffective radiator is the engine will often come back down to normal temperatures if you run the heater on the highest temperature and the fan on one of the higher speeds. I had that problem many years ago with my '88 Grand Caravan. It ran hot at anything over 60 degrees and 60 mph. As it got progressively worse it acted up at lower speeds and temperatures. I was still able to make a cross-country trip from Wisconsin to Colorado by running the rear heater once in a while. That was not a severe overheating problem; just a little hotter than normal when flying down the highway.

I don't think junk in the block is the problem; not after 30 years. What happens is casting sand gets stuck in a corner in the back of the block and often promotes corrosion of the core plug, then coolant leaks from the transmission bell housing. When that plug is replaced, there is a handful of sand, (mud) that can be pulled out first. Even that doesn't cause overheating.

It sounds like the radiator is doing its job with the coolant in it, but based on your description of getting hotter as soon as you accelerate, it almost suggests there is insufficient coolant flow. When you accelerate, more hot coolant flows to the temperature sending unit causing the gauge to rise. Check the fan to see if the belt is loose or slipping. A V-belt can be tight but still slip if it is worn. The sides are supposed to wedge into the sides of the pulley. When the belt wears enough, it will ride on the skinnier bottom and the sides no longer grab the sides of the pulley. That lets it slip regardless how tight the belt is. The same thing can happen if the belt is too skinny to start with. With low flow, you might also find the temperature of the air from the heater is lower than normal when the fan is switched to one of the highest speeds but that can be hard to judge in the summer.

Have you needed to add coolant regularly lately? If a head gasket is leaking, combustion gases will be forced into the cooling system and you would see bubbles in the reservoir, including before the engine warms up. Coolant would also sneak back into the combustion chamber so the level would go down in the reservoir. If that isn't happening, I would find it awfully hard to believe a head gasket is leaking. That just was not a well-known problem in the '70s when engines had cast iron heads and block.

If you DO see bubbles in the reservoir, that could allow the thermostat to close and lead to overheating. Thermostats only respond to hot liquid, not hot air. Air pockets in the Chrysler engines was not a problem in the '70s because the air would just sail right past the thermostat and collect in the upper radiator tank. I could see if there was enough air leaking into the coolant that the thermostat might start to close but I still would need to be convinced. If you ran the engine without the thermostat and the overheating stops, I'd still suspect the thermostat first before a head gasket. If you try removing the thermostat and it overheats even faster, try squeezing the upper hose to restrict coolant flow a little. It is very possible to CAUSE an overheating condition by removing the thermostat where there was no problem previously because without it, hot coolant will go through the radiator way too quickly and won't have time to give up its heat to the metal tubes. The clue is you'll find the lower hose is much hotter than you just found it to be.

As for flushing the cooling system, all I ever did at the dealership was to remove one heater hose from the engine and the lower radiator hose, then run water through the heater hose and port from a garden hose. I've heard some stories about plugged heater cores causing overheating because the return coolant, (which is still pretty hot), is what helped the thermostat to open. I'm not sure I believe that story but it would be interesting to see what kind of flow you get through the heater core and whether it improves after flushing. I don't think you're going to see anything come out from the block that hasn't already broken loose years ago.