Howdy and welcome!
I think 2.5" mandrel bends will suit your 6 just fine, and any straight through muffler will produce the most power. Magnaflow has good sound (deep) and Dynomax is supposed to sound good, too. 2.5" systems net ~8 peak hp. (at the wheels) on our 2.5L engines, so expect something comparable. Again, welcome!
