Re: performance ex.
Two chamber, center in/center out Flowmaster is a really good sounding muffler. I think for the sound range you are looking at, this would be the Flowmaster of choice.
I ordered mine from lamuffler.com, but Summit, Jegs, etc. all have them. Heck, you might be better off to look for a local shop though, I think Flowmaster prices are all relatively the same no matter where you buy them.
Also, don't bypass mandrel bending. This really adds to a free-flowing exhaust system.
One last point. Reconsider duals out the back. If you are hooking up a trailer, you have to turn the truck off or you will breathe in a lifetime supply of exhaust gasses. Your pipes will be completely submerged if you are launching a boat. Any trailer will be covered with exhaust stains. And, it is extremely easy to back your pipes into anything off-road. Due to these factors, I had to get a single side exit exhaust.
'89 K1500 350/700r4