On most production cars the muffler placement is driven by where it will fit and how loud they want the car to be. I have a 2005 Mustang with cats (that muffle a little) before the X pipe, and mufflers as the last item in the exhaust. My SBC 350 has mufflers about 2/3rds of the way back because there is lots of space underneath my truck.
If your SBC is less than about 325-350 HP, 2 1/4" pipes should work fine and they will be easier to fit in the chassis. With an H pipe you can usually go with a more open muffler, but maintain a more pleasant tone. X pipes often help high end HP, but they shift the exhaust tone higher than an H pipe. For a relatively low-pitched performance car tone, many cars use some type of Turbo muffler, like those from Dynomax, Summit, etc., and a good H pipe. For a little louder tone, a chambered muffler (especially a two chamber) like the Flowmaster might be what you want.