['68OrangeSunshine]

I know how big 33x12.50x15 tires are, so it's not possible to stow a spare under the frame like on C/10 Stepsides. May also be too big to fit the spare tire cut-out in the Dr side fender if so equipped.
I think the Factory once provided a spare-carrier that bolted on the inside wall of a pickup.
Easier to fab your own if you weld.
If you built a triangle of angle iron stock, with two ''feet'' at the bottom coming out a few inches to bolt down into existing deck side-runner bolts, and the apex of the triangle ran up to mate with the 1st aft upper fender bolt, that would be your base. A triangular plate would project out from there, and mount 3 lug studs to hold the rim of the wheel. If the design lets the weight of the tire rest on the floor, rather than suspending the whole weight over the floor, it would be subject to less stress.