I personally wouldn't weld them on. I've had to remove my rails multiple times for other mods. It sounds like the holes don't quite line up with the holes in the frame rail. If that's the case you can oval out the holes until they line up.

 

They line up fine it's just the one with the plate feels like the bolt just isn't long enough. As soon as it feels like it starts to grab and get tight it just slips loose.

 

It's that nut clip. Can you shine a light up that hole? Might have to try and work the nut clip out or lower. What I think happens is that you go through the nut clip first and then into the body. You should by the way be using some lock tight on the bolts when you eventually get this right. I know it's a pain but sometimes you get what you pay for and it sounds like you're experiencing a case of that right now. Good luck.

 

Don't you hate it when an easy job ends up being a headache. I was putting on some custom boat side sliders on mine and the mounting holes just wouldnt line up. They were heavy as hell and it took two guys holding them in place while I bolted them up. I ended up selling them, they weren't worth the hassle.

 

Found out its not the nut clip, it was just the bolt really stuck but it won't thread in. It will with out the rail but not with it. And it's the only one with that lil plate welded on the bracket. Just that bracket all the others don't have one and they fit just fine.