I don't see that size for mufflers or cat's. Do I just go with 2.5"?

Well, if ur going to go with a mandrel bent system then go 2.25 at the most....ur not going to gain any power with bigger piping unless your going turbo. I have a crush bent system and i still stuck with 2.25" custom pipe. Im happy with it, and you can even go 2" mandrel if you wanted to. ITs all up to you.

The 2-3/8" piping is on the car. I'll be adding a cat and possibly replacing the muffler. The exhaust system apears to be in good shape, is crush bent but cleanly done.

Ahh, i see what your saying. You have the piping and you need to know what size inlet on a muffler will fit. Your best bet will probably be a 2.5" like you said, but even if you went with a 2.25", they could make that work too. Either way, im sure whichever shop you take it too will be able to weld it so it looks pretty much seamless.