Since it's pretty obvious you can work metal here is something I've seen done on these old trucks (usually crudely) but could look nice. They cut the area the spare tire will fit in and flip it over so it's concave. then weld it back in. Careful measuring would be essential because when flipped over any slanted cut would overlap on on end and have a gap on the other. Of course the easiest would be to get the under bed spare mount. Your right on the workability of the metal on these old trucks, much thicker and softer than newer stuff so unless its completely rusted away--it's fixable. The two toughest fixes we have are where the truck has been driven with a broken link on snow chains (hundreds of knots), or where someone has tried to fix it themselves (ALWAYS stretched metal). As far as rust where the runningboard bolts on, breaks around the flange holes, brace rod ears torn off, beads cracked, tears in metal, no problem. Modern truck owners can only wish to be so lucky.