Thanks, Rick. As usual, good answer!
The primary reason the 350 and smaller blocks are less "desired" is due to the overly SMALL bore size. The intake port and valve can't "work" properly in such a bore. The mixture flies down the runner, over the valve and BAM! right into the cylinder wall. The HUGE "reliefs" scalloped into the bores at the top are an attempt to improve this. Even the earlier 400s with "performance" heads had the relief, and that's a full 1/4" larger bore.
It's true, Pontiac used large-valve heads on some 350 "HO" engines. It has been our experience, with a good valve grind and some judicious port work, the small-valve heads will make more power on top of the 350 block, than similarly "done" large-valve heads. 17 and 47 are the castings I would use for a "performance" 350P.
Jim
