Glad to see that things are progressing! I'm proud for you to have such a fine car as your first car, too.
For now and initial diagnostics, fill the radiator to about 1" below the filler neck and put the cap back on. Let it idle and see how long it might take for the heater to work and watch the temp gauge. Then, with a non-contact IR heat gun, check the temps on the radiator core at various locations acorss it, the lower hose temp, the thermostat housing temp, and heater hose temps. Most should be close to 180 degrees F, if it has a 180 degree thermostat in it. If it might have a 195 degree thermostat, the temps should be raised appropriately. Also look for evidence of leaks at the core plugs on the sides of the engine block, too. You can also check the temps of those plugs, too, for good measure. ALL diagnostics so far, just like doing a compression test of the cylinders.
Then, turn off the engine and let it sit overnight. Recheck the coolant level in the radiator, looking for a lower level of things. But at this point, a lower level MIGHT be due to getting air out of the system, too, so re-top-off and let it run about 30 minutes again. Let it sit overnight and re-check the level. IF it is still good, THEN you can proceed with a drain and re-fill with coolant.
Used to be that a Chrysler 26" a/c radiator would take 9 quarts of antifreeze to fill it up, which would then result in a coolant protection of -34 degrees F once everything got mixed up. BUT for your initial deal, I would recommend 1 gallon of green coolant in. Reason? It has been observed that an antifreeze mixture will find ways OUT of the system that plain water will not, so no need to waste a gallon of antifreeze in this warmer weather, to me.
Coolant flushes? My orientation is to NOT use a caustic coolant flush operation on anything which is older (or more than about 3-4 model years old). Reason? As the internal scale attaches to the core plugs, becoming a part of them with time, removing that scale/crud/corrosion will make the base metal in the core plugs thinner and even thin enough to fail, causing leaks. So, nothing "full strength" at this time. DO plan, before the weather turns cold, to put the car on a lift, knock out the core plugs on the side of the block, and flush out the block's coolant passages. With particular attention to the rear cylinders on each side, as that's where "the accumulation" starts, as that's also the lowest part of the engine as it sits in the car. When done with all of that flushing, install new btonze freeze plugs, 2 gallons of new antifreeze and then water, new themostat of your choice, new radiator hoses and heater hoses, and that should have all of that stuff taken care of. Watch for heater core leaks, too.
With the heat gun readings taken earlier, you can also determine what might need to be done with re-coring the radiator, too. IF the radiator is holding coolant well and the car is not getting past the 1/2 mark on the temp gauge, you might delay any radiator repair until next Spring, if desired. Your judgment call.
Hopefully, you've looked at the ATF color and smell, too. Might be good to change the fluid and filter, too. Dexron III-type fluid was the Chrysler spec for back then. Plus some new motor oil, too. Plus the rear axle lube level, too.
DO figure on replacing all of the rubber fuel line sections, starting with the one at the tank sending unit. Note the special clamp/fuel tank sender ground back there, too. Then the short section near the front and the sections at the fuel pump and fuel filter. The current fuel line hoses are old and probably drying out, which the ethanol in the fuel will make worse, so replacing them all with new-spec hoses is a necessary investment in reliable operation in the future. Might also be why it was hard to start? Figure in a new quality fuel pump and GOOD new fuel pump pushroad, too
Until you get it running reliably, you might wait to do any tune-up type work on it, other than to just get and keep it running.
You've got a neat and good looking car. Take care of it and treat it well, just as you would for a close friend. Use all of the mechanical work as a learning curve experience, too. Doing everything well, although there might be a few areas you can temporarily skimp on, by observation. When done, the efforts will pay off in a unique (for the times) vehicle that is comfortable and reliable to own and drive.
Keep us posted on your progress,
CBODY67
