I agree with Wrenchbender about the 305 heads. Some people have attempted to raise the compression ratio of a 350 by using their smaller combustion chambers, but because of the small bore of the 305, those heads are very limited in the size of valves that can be used.
I tried to make a 305 into a performance motor once. I failed miserably. After that, I decided that there was no substitute for cubic inches and started using 400 smallblock motors, which are still my choice for V8 applications.
Honestly, 55Cabover, I think you'd be much better off with a 350 and Vortec heads or even 882 heads (best of the worst...or worst of the best...whatever) and a camshaft that's designed to maximize torque and a dual-plane intake manifold. I've used both Vortecs and 882s with good results. As Builder mentioned, the Vortecs take unique parts, but when they're used together, they are real good cast iron heads.
Again...if torque is your goal, I'd look at the cam before the heads. It might be worth it to toss an "RV" cam in the motor and see how it runs before going with a different set of heads. But of course, it's your decision.
