My understanding of the NAPCO stuff was that it was designed to be bolt on conversions, put on at the dealership when the truck was new, or slapped on the truck a few years later by some farmer in his barn.
Of course, each "kit" was suited for a particular year and model. I guess the real question would be how different the '55 and a '58 are frame and suspension wise. If the two were real close (or identical) then it outta be a direct unbolt and bolt back up kinda deal.
