66 for pickup????????

If it's a completely round housing then it's probably a 9" rear, i believe the trucks came with them back then, but i could be wrong. The most likely place to find parts for this truck is in local papers, and auto/parts trader magazines if you want a decent price for the stuff, auto salvage yards with older vehicles are also likely to have stuff for it. If it is a 9" rearend, i believe you can just swap out the 3rd member and many places offer bolt in 3rd members since yours sounds like it's seized/rusted and there are many rebuild kits you can buy for the axle assembly. Good luck and have fun!