I don't know about factory options regarding the '58, but it's a common argument, probably as old as the ford vs. chevy debate.. about which looks better: a side mounted spare, or a under bed spare.
The factory option for a side mounted spare on a 1/2 ton dosen't seem to have been available till the mid 50's in Chevrolet trucks, altho many "farm trucks" have been modified to accomidate a side mounted spare, inclding my '46 1/2 ton that was rigged to accomidate a spare on BOTH sides.
The big clue to a factory side mount spare on a 1/2 ton truck would be the factory dimple in the fender.. that can help you determine if your side mount spare is a stock option, or something a previous owner rigged up.
I like the spare on the side.. especially if you've got a rear flat tire and the truck is sitting in a foot of mud!
