header size for stroked small block?

It should be 475 to 500 hp. How about the rest of the system...is 2.5" ok or should I step up to 3"? TTI said I can fit 3" pipes over the rear axle without issues. I've always heard that 2.5" is the biggest that can fit.

Also X vs H pipe? It the "X" just a hype or is it really beneficial?