With your cam and setup your 2 1/2" exhaust should do just fine, a three inch system might give you a little more up top and boost the bottom end too. Corking up the exhaust to get backpressure does one thing, reduce emissions. Not something I am figuring you or I much care about. Finding mufflers that are pipe size all the way through is hard because the MFG's like to make one muffler and then expand the ends to fit a different size pipe, the cheap ba_tards.
The good thing is this restriction is far enough back of the exhaust port that gains are small if you did fit a proper sized muffler. You could try a diesel engine muffler to get a big muffler, you probably have the room under your truck. Probably wouldn't gain much HP although it would last a lot longer as they are heavy duty. I have always liked the late sixties early seventies big block Chrysler mufflers, very quiet and have 2 1/4" through hole size I believe. Big and heavy mind you.
“She may not look like much, but she's got it where it counts, kid. I've made a lot of special modifications myself.”
— Han Solo