My truck is a Regular Cab Short Box 1500. I'm working on a frame off restoration (lots of rust in this area) and LS swap. There is an undamaged (and mostly rust free) frame for a 2500HD Regular Cab 8' Box for sale locally. I went to it twice with the idea of replacing my SB frame with the heavier RB frame. Most of you are aware, and I became aware after measuring twice, that the frame tapers from the same width at back of the cabs to the same width at the end of the frames. The SB frame tapers at a sharper angle than the RB frame so at the 6' point the RB frame is wider. I threw away my diagrams with actual dimensions but sketched up one to show visually the difference. Something as basic and simple as the rear SB box mounts would be in thin air because the RB frame is wider at that point. And that is just the beginning of dealing with the width difference problems. Add to that the spring purchase on the axle would not be the same width as a SB and not the same as a RB.
I hope this helps.
I hope this helps.