OK. Might be a little more difficult than I thought, but it can still work.
I thought the sliders would go around BOTH sides of the frame, not just one.
In that case, I would still do the same as above, just weld a plate to the back side of the frame, for all three peices. Even if it's just some 1/8". That way, you'll never have to worry about having to balance the sliders on your knee while you're unbolting them, or worrying when it's going to bust your face open.
You don't have to solidly well them all. Just run a couple beads around the plate. That'll be enough for it to hold.
Make sure you do it when the slides are ON. That way, you don't have to worry about the bolt not lining up with the holes, and the holes not lining up with the nut.
Basically, put it all together, then weld the nut and plate in place. It'll be a bit of a PITA now, but you'll be glad you did it in the end. I did it for my custom crossmsmber and it's awesome. 3/8" thick steel is HEAVY.