My plan is to put a 1993 Club cab, on a regular cab, 8 foot bed chassie, with a step side bed, Short bed,
Will the cab even fit without modifying the frame?
Probably not. The 73-93 full size frame has a kick up behind the cab. On a standard cab frame, the kick up is closer to the front then it is on a club cab frame. I don't believe there is enough room between the cab floor & the frame to accommodate that kick up when going from a standard cab to a club cab. The club cab will be sitting higher in the rear. A 2"-3" body lift would probably solve that problem though.
I suspect the step side box will be hanging off the frame by at least a foot, and it will be surprising if the rear wheels end up anywhere close to be located correctly.
I believe you need to find some trucks and do some serious measuring. That is probably the only way your going to get a real answer. We can't even agree on the length of the different boxes, and your throwing different cabs and different frames into the mix.
Gene