[AZKen]

It is a VERY LOW RPM motor. Built for torque. To buy one you must realize that. If you want a hiway truck, there are many choices. Many are very expensive. I don't know how much you can do. I don't know your skills, tools, experience. The suggestion are coming in from members. It all takes work and some time. Maybe just a rear diff gear change will work. It worked for ED. That would be your first step. If you are going to take it to have it done, it will be way cheaper to find a 12 bolt Chevy donor to swap in. The only way to be sure of any ratio is to take off the cover and read the numbers on the ring gear. Please state if it is a 1/2T or 3/4T.
My 1960 GMC 1/2T had a Dana 44, Jakes 1960 GMC 1/2T has a Dana 44. Yours does not. Don't know what it is yet, corporate 10 bolt? or a drop out? or other. 1/2 T or 3/4 T? Could be stock or could have been swapped already. I can't see your lug nuts to tell if yours is 1/2T or 3/4T.
Your truck seems to have some custom cab features.