It sounds like he drilled all of the holes at one time with the nerfs off the Jeep. This would be an explination for the holes being off. So i would think that in order to make them work he took off enough metal to make the self tappers go in, this, I think, would make the mounts a little weaker than if the holes were drilled one at a time and the screws went in perfectly. I would use your own judgment, if you think you took off too much with the drilling then you need to beef up the mounting ,and if you are even asking this I think you should make it stronger. Now as far as welding them goes, that will make it hard to get them off if you ever need to. Maybe you could just re- the holes through the nerfs, and use an all together bigger screw. That might just stiffen them up, if the screw is a tighter fit into the hole you made. Just an idea. Joe