I think you need to look into if you received the right one or not from Chassis eng. The 3/4 ton and 1/2 ton frames are basically identical other than the 3/4 is about 8" longer. The width is identical along it's length as far as I know. Let me put it this way, my truck had a three quarter ton frame under it, I swapped it for a 1/2 ton and all the bolts for the bed and cab went right back where they were, I don't remember a bit of difference. The bed bolts down right thru the top of the frame rails at the rear, how could the rails be wider apart? As I remember there was no mod to the rear axle (a 1956 passenger car) that I pulled out of one and put in the other frame. So even the springs where in exactly the same place.

I think you have found the common issue with these parts from many vendors (not picking one this one in particular) they DON'T FIT!

Brian