Papa7, what you're really looking for are higher gears. Unfortunately, there's really not much available for the Eaton rear other than the 4.10 option from the '68-'72 3/4 tons with V8/automatic. Those are starting to get hard to get due partially to the demand, but also to many of them getting used up by the dirt racers in the '70s/'80s. The only other option is to change out the entire rear for a later model with a better gear. A Dana 60 from a late '80s/'90s Ford or Dodge van, usually with 3.55 gears, is just about the right width for an AD truck.
County Boy, since you have a GMC, that gets into a completely different animal than the Chevrolet. For the most part, GMC used the full float version of the Spicer (Dana) 60 in 3/4 tons. That will make things much easier for you, as the 60 is still being used to this day, mostly in vans and some 1/2 ton GM trucks. Due to the popularity in trucks as well as certain MoPar passenger cars, as well as the racing and off road communities, parts are readily available. I would suggest that you check out Randy's Ring and Pinion. You should be able to find any gear ratio you want there. Be warned, you may have to change the carrier depending on what gear you go with.
