I also have a '92 F250 and at the time I bought it 2 years ago the performance at highways speeds was similar to yours: a gutless wonder. About a year ago I installed a used ATS turbo onto the motor. I was towing a car (on a trailer) from Eugene, OR to Chico, CA when I stopped midway to pick up the turbo unit in Ashalnd, OR and then drove over to Klamath Falls, OR to have the turbo installed by a good diesel mechanic there. On the way to K-Falls the truck slugged it out along I-5 doing, if I was lucky, 55-60 MPH. After installing the turbo I left K-Falls and headed south, facing a long grade to get out of town. Pre-turbo I would've been lucky to maintain 35-40 MPH towing that car. The turbo effect is nothing short of magic: I found myself having to throttle back 1/4 way up the hill just to maintain the speed limit (55 MPH)!
So I can unequivocally say that putting a turbo on your '92 is not only feasible, it's a virtual necessity if you're going to tow anything. Highway mileage has remained about the same unloaded (14-15 MPG) but towing mileage went up significantly (from 8-9 MPG to 11-12 MPG) and going over passes at higher altitude you can really tell the difference in performance.
My (barely) used ATS turbo cost $500 + $300 to install...and worth every penny :-).