I had a "freind" one time that brought a set of heads to me for his 76 Chevy 350. I put them one without paying close enough attention to them. Until I tried to set the timing and make it idle right. Tinkered with that truck for over a year before finally decided something was wrong. Pulled the heads back off, one was a 350 head, one was a 305 head. If you set the timing where it would half-way run, it wouldn't start without grunting at you. If you set the timing where it would start when it was warm, it didn't have any power.
The factory did use 305 heads on some 350's, but they had recessed piston tops and I guess they did that for emission purposes.
