Ok, back to the original question.
My 5ton frame is 9x3”od, by 5/16” thick. Assuming Tobys dimensions are outside, and correct, in the grand scheme of things I wouldn’t call 1” taller and 1/16” thicker x2, way heavier. If your already dead set on 5ton axles, that little bit of weight doesn’t sound worth tearing a second truck down just for a small weight savings. Why not just stay with the pickup frame, if the trucks just going to be an old school monster truck, and not see any real abuse? I think even 2-1/2 ton axles would suite your application, but if it where me I would go with csmdavis idea and just throw the Ford cab on an m939a2 tractor and call it excellent. Then again I do weird things...