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I would use a full size tire, especially if you have a limited slip diff. If you have wide off road tires you can get one that's the same diameter but more narrow. This assumes they're a reasonable diameter... I don't think 44's are going to fit under there...
There's a bar that hooks up in one corner, drivers side front and goes cross ways to the passenger side rear. It's hooked in the front, the tire's put on then you lift it in the rear and attach it.
Mine didn't come with one under the bed. So my cousin and I mounted one from an import truck, cant remember what make, and welded it up to the frame under the rear of the bed. Kind of nice, just pull the crank out and crank it down. Unhook the chain from it and done. Works great.
I finally got my 86 last night and it's got the underbed carrier with no tire in it. I can take pictures later so everyone can see. It also has the rear tank.
These trucks came with a standard spare tire carrier under the bed. If the truck came without a spare tire being ordered from the factory a two piece block of wood was installed in location of the tire. This was a simple carrier with a bolt on one side and an eye bolt on the other.
There were two optional spare tire carriers making a total of three with the standard one.
The slide out spare tire carrier was and option, also located under the bed. It had a handle that you could rotate, unlock and pull the spare out from under the truck.
The In-Box spare tire carrier was located in the bed and bolted to the side of the bed either on the driver's side left front, or passenger's side right rear.
I have, well had the in-bed mount on my truck. You can clearly see the holes in the bed for the bolts. A previous owner was nice enough to take the actual mount bracket out, but he was nice enough to leave the bolts.