Years sound good to me 75-79. My 75 I converted the t-case to part time, and added lockouts to the 60F. Dad had a 77 3/4 ton fulltime truck with a 400 4 sp. 7 miles to the gallon no matter what. It loved to tow, and high rpm's. It would eat my dad's buddies 454 auto any day towing. They worked together tiling, hauling the machine and backhoes, hay, cattle and whatever else. Dad had lots of carb, and ignition problems, traded it in late 92 for a CTD( the 93 I have now). The 77 had 66,000 original miles on it, with a beatiful body!! Knowing what I know now, with a MSD box or Jacobs ignition, and a new edlebrock or holley would have made that truck more reliable.
Michael