[DieselSJ]

87 454. Mine was 3" from the factory - 2 1/4" off the manifolds into a 3" cat and went to dual 2 1/4" pipes after the muffler. I swapped in a new 3" magnaflow high-flow cat and a huge 3" in/out magnaflow muffler and 3" over the axle and out the back. You can hear it just enough at idle to know it isn't stock, no drone or resonance on the highway (in fact it is dead quiet on the highway). Picked up a consistent 1.5mpg over the stock setup.

In your case, either a dual 2.5" with a x-pipe or a single 3" would work great, IMHO.