Although the assembly manual is sold as for 55-59 trucks, it does not cover the things that are different as the truck design changed over that period. My assembly manual is at least 25 years old but I don't think the manual was revised over the years. Sheet 25 section 2 in my book has a detail drawing of the 3200 frame for the shorter bed you desire. Note there are four 7/16" diameter holes dimensioned along the rear portion of each frame rail. Those are the bed to frame mounting points for that shorter bed and you will likely find holes in your 58 frame match those points except for the rear mounting points. Simply cut your frame to the length dimension shown and drill the appropriate 7/16 holes at the rear and you should be good to go.

Mark