The small intake and exhaust ports on 305 heads won't allow a 350 to breathe enough air to take advantage of the added displacement. If you were building something other than a performance engine that would normally not see over 4000 rpm, then it would not matter so much. But heads are where the hp and torque comes from in a performance engine, and you want to match the heads, cam, pistons, and induction system for optimal performance. 305 heads are simply a mismatch. Now if you were talking about building a hot 283, that would be different. I'd say go for them.