Chevyguy:
It seems like you have a 60-62 bed that you are trying to put on that 55 frame. 60-62 had a wider frame and the rear sill "ears" are spaced about 6 inches wider than the 55 frame that you are using. Quite a few other things different also, but that rear sill mounting will be your biggest problem. Check
http://www.mar-k.com/bed_side_interchange_gm.html for more information. Even if you move the brackets inward the 3 inches on each side, it will position your bed about 3/4" higher and a couple inches too far to the rear compared to stock.
Mar-Ktech
