340 slight dieseling when hot and hard start

IF boiling fuel is what the problem is, there is zero need to block the crossover. It's there for a couple of reasons. Quick cold weather warm up and better fuel atomization. So if it's a street car, you probably want it left open. All you need is the THICK (1/4" or more) carburetor insulating gasket under the carburetor that everyone forgets about and even a carburetor heat shield. I recommend both.

You need to properly diagnose it first though, because you have the car, not us, so anything posted here is just a guess. Otherwise, you'll continue to chase your tail.