[Sirrotica]

I've done both, turbo mufflers under the back seat with full tailpipes and it droned like crazy on the highway, not at all pleasant to drive.

I also used a transverse muffler from a 87 buick turbo regal that other than being too narrow and needing some pipe extensions to mate to the Pontiac tailpipes with extension pipes, fit quite well and sounded good behind a modified 455 with headers. I made both systems out of 2 1/4 inch pipe and didn't feel it needed to be any larger, car ran really well, sounded good. I never worried about dragging mufflers over speed bumps either.

If I had to do it again, I'd go for the transverse muffler, same as 77 Canamman said.