the 255s will fit fine, go with 265s, they were an option tire on 88-98s and fit perfectally.
your getting to technical with all this "rolling distance" and "tire weight", your only gonna get a difference if you going from say 31s to 35s and up. just go with 265/75-R16, nice size tire, very common and easy to find vs 255/85-R16.
Your not gonna get much better than 19 mpg with a 3/4 diesel just with tires. Go simple
Edit: I had a 95 3/4 burb with 265s, rpms sat right at 2 grand at 60, still got 15/16 mpg with 4.10s. it was also a 5.7L lol