55.2-59 long bed to short bed frame question

[Kevin_C]

So I just finished up shortening my long bed to a short bed and took the 9&1/4" out and welded it back up. I was about to start installing the flatout rear brackets and noticed the instructions were for a short bed and have a center line at 36&1/2 from the rear which puts the rear end too far back. when I measure from the front mounting reference hole it appear to be 3" longer. Can someone confirm how much longer the rear frame rails are on a lonebed over a short bed. Also how much needs to be taken out of the rear bed sides. I currently have a 3.5" gap between the fender and the rear piller