My 1993 SCSB Stock had 5.7L NV3500 MG5 10bolt 8.5 ring with 3.08 gears and 235/75R15. Got 18 MPG empty truck.
Currently, it has the stock 5.7L swapped NV4500MW3 with Gearvendors under/overdrive 14bolt SF 9.5 ring 3.73 gears and 245/70R17 aluminum Yukon rims from a 2007. My best now is 19.5 empty truck.
That gearing is going to make a big difference. 3.08 is tall gearing and was probably the main reason you got 18 mpg. My 5.7 has 3.73 with a stock 700R4, so it only gets 14 mpg.
On top of good mpg, a really long range is interesting to me. I remember my old 4.3/5spd got something around 400 miles per tank. It was a short bed, so it had the smaller gas tank. I've always wondered what a 4.3L, 5 speed, long bed with the big tank and 3.08 gearing would get. It would be cool to go 500 miles between fill ups!