340 with edelbrock heads overheating

your overheating problem is probably caused by tight piston to wall clearances. it will improve as you put more miles on it.
another thing : check your piston to head clearances . the piston shouldnt have touched the head anywhere, especially in the chamber . i believe it may be contributi
ng your engine to overheat , especially because you said that it didnt overheat before you switched heads. cometic gaskets are available in thicker sizes, although more pricey. you dont have to have the heads nor the pistons machined if you find that you dont have enough clearance, just use a thicker gasket.