If the 6.2 is still in good shape, 18-22 MPG is quite possible. My '82 is trashed (made 65 HP on IdahoRob's dyno) and still gets 16-17 MPG. And that's with me flooring it everywhere (because 65 HP). I used to get 19+ MPG when it ran right.
I do like the body style of these trucks. Classic good looks.
Oh, and yeah, don't straight pipe it. I did it back in high school and it was so loud I couldn't stand it as a young punk kid. If I'd driven in to town, I guarantee I'd have got a noise ticket. No idea how that engine is so loud.