Spare tire mount on 1979 GMC 35 Crew Dually

[wolverinemotorworks]

Can anyone tell me where I am supposed to mount the spare tire on my truck? For some reason I thought I saw one mounted underneath the rear of the bed between the frame rails. It is a 16" dually rim and tire. It was mounted by the PO on the front of the truck, but there is no way it fits in my garage with it up front. I want to keep the bed clear for hauling wood and I also want to install a receiver hitch class III. Any thoughts? Here are some pictures of my stuff.