Wow I haven't been on here for a while and you're moving along at full clip! Good progress, really just removal of a single one for them crossmembers would count as good progress in my book. Tho given how open your access to the frame is, I'd have been very tempted to go for the blue wrench instead of the grinder. Also I've had better luck with just a regular punch and a 4-lbs engineer's hammer while driving them rivets out, few good swings and they're out, it's less fatiguing to me than the air hammer and it's a good workout to boot... Well, and I just like hammers a bit too much
Btw backing plates like the ones you made for your crossmembers (but with stainless nuts) is how we bolt our spares to the bed rails in one of our trucks. The better half was actually supposed to tack them in way back when, so they don't fall off every time the wheels get pulled, but knowing her I somewhat doubt that actually did happen - guess I'll find out next time I need a spare wheel
How worried are you about frame twist? The factory crossmembers are stamped not only because it's cheap, but also because it's not torsionally resistant, meaning when the frame twists the crossmembers will elastically twist with it. Tubing on the other hand is very stiff torsionally, so if the frame goes to do its twisty-twisty the crossmembers will not like that very much. Unless of course the U-haul box makes the frame so stiff there's like zero twist going on to begin with?