I would be careful swapping heads from other SBC such as 350 or 400 because I believe the 305 bore is only 3.75" while the 350s are 4.00" and I believe the 400s are 4.125" diameter. This means that the valves from a 350 or 400 will hit the sides of the 305 cylinder wall! You need to look for a 305 HO heads or buy Edelbrock 305 heads.

Don Shillady
Tetired Scientist/teen rodder