I went through this same thing last summer with
my Duster! I have a mild 360 currently as well.
I debated and went back and forth for weeks...
deciding on what to do. Since mine is just a
weekend summer cruiser (car show/cruise nite/ice cream runner type Mopar)...I went with a header back Jegs(Pypes) full 2-1/2 inch kit which is a nice kit IMO.

All aluminized mandrel pre-bent tubing with
(optional H-pipe) which I got.

I wanted to go the full exhaust route, with 2.5" stainless steel tips. I had a local
muffler shop do the work,because I wanted everything welded properly. They had an easy time until they assembled the 2.5" tailpipes next to the factory gas tank. It took them quite awhile to get the fitment just right on the pass. side pipe...between the tank and leaf spring.
IT'S TIGHT THERE! The system turned out great though,sounds awesome with Flowmaster 2 chambers.

So....if you are thinking of going with a 3 inch system out the back...think twice about the gas tank issue! Good Luck!