our stock exhaust is i believe 3 inches? you can do what i did and hollow out your cat with a whole saw and shoving a piece of pipe through the middle, that will make it look like you still have your cat on (great if your state does emissions inspections for diesels) but you'll really just basically have a "case" around a straight piped exhaust
The state I live in does require emissions on these trucks and mine is a 96. When I bought it, it didnt have cat they required one here to pass emissions. Its just a visual inspection though meaning all they do is look to see if it has one, not wether it does anything or not. So before we put one on it we took a crowbar and a sledgehammer, and "adjusted" it by removing the guts so its just a cat shaped pipe and it passed emissions just fine.