The tail housing from the one you bought will not work--does not have the proper shifter mounting position for a GS, and the rear trans mount pad on the bottom is about 1/2" different in location, so the mount bolts won't line up with the holes in the crossmember.
You can use the main case though (cast iron part). Only difference is the location of the casting numbers as was already pointed out. The casting numbers moved to the lower location in 64, so the one you bought is pre-64.
Your original GS aluminum tail housing can easily be welded. Just clean it really well and bead blast it before you take it to be welded.