The 2 1/2" system on my '67 was bent to original specs, but with an "H" pipe and the shorter body DynoMax SuperTurbo's - I also have HO manifolds. After a lot of looking, I found some data that indicated the DynoMax SuperTurbos can provide zero flow restriction up to abot 185 HP per muffler - that works out about right for my ~360 HP 400 engine. They make a great sound, although there was an obnoxious "drone" around 1600 RPM at light throttle (cruise in O/D at about 50 MPH). I had noticed the same thing on another car I put those mufflers on and that issue seems to have gone away as I put miles on the car. Or maybe I just learned not to drive at that speed/load combo :-) I think the drone is getting less as I drive the GTO, too.
The headpipes are 2 1/4" in order to have enough clearance where the frame, starter & suspension monting points make for a tight fit.