BBK makes a twin 63mm throttle. FRPP makes a single bore throttle, but it requires some grinding ("blending" by the catalog) on the intake manifold.
As far as the X-pipe/Nascar thing, someone has been reading Bassani's ads too closely.
But in general(key word here) a H-pipe will work better with a matched (also a key word here) cat back and a X-pipe will work better with a straight though muffler(ditto).
Why, is complicated. Back preasure? Actually Delta P. But for 1 or so hp I would say, just go with the sound you like best.