I have been dealing with a vacuum leak on a fresh 383. Thought the intake wasn't sealing properly and while looking for the leak with a can of carb cleaner found the leak was right around the exhaust crossover. When I spray carb cleaner directly in the opening in the head around the crossover, it will stall the engine. I will pull the head and check it later this week. Do the iron heads develop cracks in the heat riser/crossover area?
Seems like that may be whats going on...