Go with 2" from the manifold to the muffler and 2.25 is basically just your tail pipe after the muffler. It is effective and fairly quiet, especially if you put a high flow cat in. That is what my girlfriend has on her 86 efi stock and it is open rev with a strong bottom end and a good low tone that is medium for volume.

i run 421 hooker header, 2.5" pipe (only about 4"long <img src="/forums/images/graemlins/notooth.gif" alt="" />) to a magnaflow(dont know what model)and maybe 1.5' tail pipe. SOUNDS HORRIBLE, kinda like a rice rocket and it is extremly loud (it was a last minite fix that kinda worked)

i am having a buddy at a shop in town return a favor and do a pro install job, prob 2.25 all the way off the header and just the magnaflow. Now i have bumped compresion, stage 3 comp cam (i belive its a 230* at .445 lift), port & polish, and other head work. Est HP now is 150-160, and that is with a stock bottom end with alot of time spent using a o-scope and tweaking the sensors for maximum fuel "basically", ther are other things you compromise for but its something to be explaned and varies engine to engine and climate to climate. Also with the set up peak rpm range is 3000-6000 rpm, below 2500 has nothing(im thinking thats partly due to the exhaust but not entirely obviously) and it flatens out hard at 6000.