I just installed the Flowmaster FlowFX mufflers with H-Pipe and 2.5" piping.
Before I had Pypes Violator and they started to get on my nerves... too loud and not as deep sounding as I think the GTO should.
https://www.youtube.com/watch?v=d1iAc0Ul8ss&t=1s
I also installed some Dynomax 3" tips. I wasn't sure if I should get 3 or 4" but in the end I'm happy I didn't get the larger ones.
Car is a 66 GTO with 462cui and a mild HR cam.