After reading thru, I'd say if you have the $$, build the shortblock, and then build the X heads to run on it. What is the part number of those pistons? I'm guessing they are rated at 13:1 with certain heads. With the high static ratio, and small cam, you could run either set. The valve size will be more than made up for with 3 points of compression. You probably will have just as much trouble with detonation with the polished Js, than the Xs, because they arent designed for pump fuel at that level. The cam will just make things worse. That's why I say get the shortblock together first, at least to set the deck heights, and then chose the heads that will do the better job. I would truely re-think using pistons that will yeild anything over 10.5:1 with any iron head. And at that level, cam choice will be critical to stay out of detonation. It's not static ratio, so much as dynamic compression that really gets you in trouble. Dynamic takes into account the closign event of the intake valve in relation to crank position, and static ratio. If your dynamic ratio is higher than 9.5:1, you will detonate on any pump fuel I can get.
As an example, I plug in the specs for 74cc head (stock is around 71-2cc or larger regardless of factory specs, then polishing), .030 over bore, an 18cc dome (guess on my part for piston), a typical FelPro gasket, and a Crane with 308° duration and it spit out 11.8:1 static, 10.94:1 dynamic. That's 110 octane C-12 for fuel.
