if you want the look, yea go for the bigger tips ontop, no biggie. you could always hammer out PART of the inside of the muffler
..that what rust has done to mine and it sounds pretty great, Cutting out the muffler and putting your own muffler could work, get an import oval (one that looks like a magnaflow or flowmaster) unless you want to sound like a ricy piece of junk. the oval cancels out the annoying tones, a circle muffler doenst ted to. there are several types of dual out oval mufflers, i think somebody has one that is has an in and out on one side and an out on the other, then just run pipes to the outlet. relativly cheap.