well, the seller lists it as having an OD trans. so either it came with 700R4 or it was installed later.
I gotta agree, with OD, 20 MPG means it's a short rear ratio, I'd pull it back to at least 373 or 342, if you're not towing, and just want MPG's
My 92 6.2 is just phenomenal with a 373 amazing power off the line, and it'll rev up and pull strong through 80mph. and I have the stinking computer controlled 4L80 that OD will not work in!!!
like that, my truck gets 19ish mpg. so, if he's getting 20 with working OD, he's got SHORT gears, or small tires.