1971 gmc gear bound!!

[ookla]

Im trying to get better top end speed and gas mileage out of my 1971 K2500 gmc-canada made truck. It has a pto dump bed (dealer aftermarket-I think)as well as being 4wd, so I thought it would be easier to change the differential gears instead of changing out the gearbound 4spd and transfer case that's in it (and not sure a higher geared set would be right speed for my dump setup). Im having trouble figuring out which gears are in it. I believe the front is a dana 44 and the rear is a dana 60 -10 bolt styles. markings are : front:
C81256 and 44 5F on differential housing, and rear: C80255 and 60 on differential housing. I think I could get different gears for front and rear--say 373...? but don't know how to figure out if that will work or not??? anyone have any input? thanks