Most, if not all, 3/4 ton and 1 ton GMCs from the '60s up to the '70s used a Spicer/Dana axle instead of the Eaton rear that the Chevy used. Does the rear end that you have found have a drop out 3rd member? If it does, then it is an Eaton and should work. If not, then it is a Spicer/Dana (most likely a 70) and willnot, unless you swap the entire rear.
Side note, most Chevy trucks with the Camper Special package were also equipped with the Spicer 70. They basically used the GMC leaf spring chassis.
