On my '91 I have true duals with Tri-Y headers. The exhaust is 3" from the collector to the muffler and 2.5" from the muffler back. There is not a kit for this; not that I have found. It has to be custom, but custom doesn't have to be expensive either!
On the crew cab with the TBI it is tight on the RH side for the exhaust, between the transfer-case and the frame rail! It is made even tighter because the factory fuel filter resides in the same area, but it can be done! For me, I am going to redo my exhaust and I am probably going to step back down to 2.5" exhaust for the full length: dual 2.5" exhaust is good for about 400 HP and the smaller diameter pipe will quiet down the exhaust noise a little.