340 slight dieseling when hot and hard start

Where is the timing and what do you have for a timing curve?

How hot does the engine run and "don't tell me" the middle of the factory gauge

Vapor lock is always a problem with this "new world order" fuel. Best thing I did for the 67 before I went with EFI was to

Install a rear mount electric pump

pay attention to tube routing

Install a vapor return system using Wix 33040 / 33041 fuel filter

add a carb base insulator

You might consider blocking the exhaust crossover

and CHECK that the exhaust butterfly if you still have one is opening