Here is one I made for an '86 I restored a few years ago.
I used two sections of square tube as rails and mounted it to the top of the runner at three points along each rail.
You could also run oem style rails like these, and mount the rack to the rails.(Will put up a better pic later today)
I make them out of either 1/2" or 3/4" galvanized electrical conduit. The conduit is about the same weight as aluminum, but costs only a fraction of the price. I think the cost of the conduit per rack is under $10.
The first rack I made has ben in use for almost 15yrs now, and has held up fine.
While the racks would easily hold a spare 35"(or bigger), getting it up there would be a PITA, so you might consider some type of swing out tire carrier.
3/4" rack on my '91
