Sounds like you have a 1/2 ton rear sill instead of a 3/4 ton rear sill. The frame end mounting bolts on a 1/2 ton are about 44-1/4 apart and the 3/4 ton frame is about 33-3/4 apart.
GMC made a 1/2 ton long bed, but Chevrolet only made 3/4 ton long bed trucks in 51. Your bed may be from a GMC
The frame extends under the rear sill mounting bracket, but not under the rear sill.
Mar-Ktech
