Help- which truck indecision!

[D13]

in the market for a driver, 70+ Michigan miles a day 5 days a week, 3 miles dirt 20 miles highway 10 miles city rest country. Really like 81-87 GMC and Chevy full size. looking for reasonable fuel mileage and long life, and not a 4wd. Already hav a 99 NBS tow beast so that's not really an issue. Found several possibilities and would like input.

84 1/2 ton 250 6cyl THM350 100k probably 3.08 or 3.23 10 bolt so could put 2.73 in it. Beleive this would get 17mpg or so as sitting, would look at swapping in 2004R and getting close to 20mpg. $3k + trans
84 1/2 ton 6.2 diesel auto 100k would this get 18mpg? $5k Anyone know what trans and rear end and ratio it is likely to have?
87 3/4 ton 5.0 probably a 700R4 60k I would bet 15-16 mpg probably 3.43 14-bolt so not much more highway gear available $5k

All have decent to great bodies and interiors. I'm assuming that the tires etc are in eqivalent shape and that I have to add a posi to whatever I get. I'm very familiar with the 6cyl, SBC are common and I know nothing about diesels.

All opinions welcome!