2.25" would actually be fine for a stock 305 or 350, but I would stick with 2.5". I have a Pypes header-back 2.5" system with an x-pipe, Pypes RacePro mufflers, and GN style exit (45ish degree angle behind the tires). It's not the quietest, but it's surprisingly quiet for the mufflers being basically straight through. It gets pretty loud with the loud pedal though. You can find a recent video of it in the exhaust sounds thread in the gallery, there are a few other vids of the exhaust I have on YouTube once you follow the link from here. The engine in mine should be in the 375hp range.