Short Bed on Crew Cab 1 ton frame issue

[DBear]

Hey guys...just finished shortening my 78 K30 Crew frame for a short bed conversion. Everything looks great....Except, the bed sits low on the rear of the frame, but sits at the right level in the front next to the cab. To get it level, the rear has to be raised right at 1 inch at the furthest rear mounting hole. I doubled checked and checked again my frame conversion and everything is square and level. The bed came off of a 84 or so shortbed 2 wheel drive. I measured the bed mounting flanges underneath the 2 beds, and the 78 k30's were just at a 1/4" taller than the 84 c10's. There still is about 3/4 of an inch unaccounted for. My only other guess would be the rear frame sections are taller on the 1/2 ton trucks vs. the 1 ton 4x4s?? Anyone else have this problem or are in the know of this situation?? Thanks in advance.