"Definitely seems overkill, but if it makes you feel more confident then its worth it. Those look kind of expensive, and the money might be better spent having a qualified welder come spend an hour going over your welds and fixing any that they think need it (and show you why and how to do it correctly next time).
If you want something you can use more then once, check out Rivnuts. They install like a rivet, but have a threaded insert so you can put a bolt in it. Great for body panels and other stuff like that and they are designed to be installed with a blind side of material like the ones you posted too."
Thanks for the input! Boxbolts are about $5.00 each, I would use 16 o the sliders so not too expensive. Brody welded them on, not me, I wouldn't trust me.
Rivnuts are cool, but I'm never removing them (they're welded on anyway) so that's not a consideration. I'm considering the Huckbolts because xventure uses them, but I'm open to finding better! Xventure actually doesn't weld the frame, just huckbolts.