It's an age old issue with these trucks. There are many posts on this site regarding better highway performance and numerous choices. There is the factory overdrive, Muncie SM319 with the Borg Warner R10H overdrive, a later Saganaw SM326 with a Borg Warner R10Q overdrive, numerous later model Borg Warner T5 overdrive options, 3.38:1 differential gears, 3.07:1 Spicer differentials offered on GMC trucks of that era. As mentioned, make sure your steering and brakes are up to the task.
Fred
