360 running on 4 cylinders

Here's a thought; IDK how to apply itto your situation, but I just thought it worth mentioning;
when the lash on a single cylinder tightens up as the temperature increases , and either valve fails to close, or closes waaay too late, the engine stalls.
When you try to restart it;
If it's the exhaust valve, the engine will pull air in from the exhaust pipe, which has no fuel in it, and it will be impossible to restart.
If it's the intake, the piston will pull in a fuel charge ok, but then on the compression stroke, push a bunch of it back into the intake, Then when it lights off, the fire goes back into the intake and pops it off in there, and it still won't start. But when it cools off, it starts and runs normally....... until it warms up again.

Like I said, In your case, I don't see this as applicable.
But now that you have experienced this with two different carbs......and the wacko manifold vacuum, IDK, it's becoming plausible. I would at least do a compression test on those 4 cylinders, first cold and then right after it stalls.