Have a '91RS, 305, 5spd, stock with open airfilter (nondrop type) and a 160 therm. With a really nice dual exhaust (no cat) with cross glass packs in back. But.. from the front subframe(?) forward it has the original, returning to a single pipe appx. 12-14'' before the Y. Would it help to eliminate the y pipe and run straight dual pipes? What about a 'H' pipe? Also would 'shorties' be a bolt on?

 

I don't think it would be worth the effort to eliminate the Y-pipe unless you get rid of the stock manifolds along with it. The general consensus here is that the manifolds are about as restrictive as the y-pipe, so I suggest you get a set of shorty headers with a Y-pipe if you just want flow, or without if you want true dual exhaust. There will be some fabrication required either way to connect your existing system.
As far as an "H" or "X" pipe, it has been done but I don't know how much ground clearance you would lose.
And don't expect huge performance gains from this mod with your current engine in stock form. However, it will improve most any engine mods you make in the future.
Hope this helps...