65 273 Carter AFB won't start

Back to basics
do a compression test.The first shot should be at least 50 or 60 psi.But keep cranking it until you get two consecutive same or near same readings, even if it takes 10 compression cycles, and don't stop in the middle.
Then squirt a small amount of oil into each cylinder. Wind her over for a good 8 seconds, and do another compression test.
Report your results.
If an engine doesn't make good compression, it also won't make proper vacuum, and so the poor old carb can't do it's thing.
If you can't get decent numbers; lash the valves, and try again
Gap your plugs while they're out.

Taking another tack
A brake booster is a prime culprit. It could have an internal leak and you might not be aware of it; while idling clamp the line off, or otherwise plug it. The idle rpm and quality could briefly flutter and then it should return to whatever it was doing prior.
PCVs are another source of trouble. At idle, they're supposed to be a fixed orifice device. Old dirty ones might not be. If the idle vacuum is too low, they will mess you up.
And the sneakiest things are the emulsion tubes in the idle wells. You have to take the clusters out to see them and clean them. While they're out, this is a good time to blow out the idle circuits. Cover the carb with a rag, remove the mixture screws , and have at it. The air should come out of the now-empty cluster pockets, and the rag is there to keep the first slug of fuel from hitting the underside of the hood
And finally....... make sure the secondary air valves are closed up tight but not sticking.
Gaskets and such,you know about. Testing for vacuum leaks,you probably know about.
Good luck