Just from what I read in the manual, the '58 is the same as '55. The mounting stus should be part of the frame/axle on 1/2 ton. There's a possability that the shocks have been replaced in the past, and original studs replaced with aftermarket mounting bolts similar to what you describe. I have run into this on later model vehicles with aftermarket shocks. You may need to use an aftermarket mounting kit, such as AK32 to fit the shock to the truck.
