Question about exhaust tail pipe size.

[Second Series]

I’m redoing the exhaust on the ’88 k2500. When I got the truck, the pipe exited in front of the rear tire. I want to bring in out rear of the rear tire. The pipe is 3” from the y-pipe to the cat, and to the muffler with the old exit being 3”.

When I search on-line for this specific vehicle I’m seeing a 2-1/4” pipe with 3” inlet. Why? Should I get the 2-1/4” pipe with 3” inlet, or get a 3” pipe? How was it from the factory? I saw a couple trucks in the scrapyard that had the 2-1/4” pipe, or dual mufflers, but no 3” tail pipe.