1/2 ton vs 3/4 ton frames

[Buick455]

From everything I have read the 1/2 ton and 3/4 ton frames were the same but they are not. My 1/2 ton 86 frame web width is narrower in the rear section in two spots than the 87 3/4 ton. I was thinking of using the 1/2 ton frame as the cab I am using is sitting on it but as I am springing it for a 3/4 ton I guess I am now using the 87 3/4 ton frame. The difference may not be that big of a deal but I might as well just go with the heavier frame as the work isn't really that much more.