340 runninng lean and overheating

Speaking of coolant... the OP could still have air in the cooling system. That can make the engine run hot and it will boil more easily. Park the car with the radiator high and refill. It may take a few heat cycles to work all of the air out to the top of the radiator.

OP, do you have an overflow tank on the radiator? Or just a drain hose from the radiator cap?

And, be aware that with headers, they directly radiate a lot more heat into the engine compartment than stock cast iron manifolds.