Many of the Xj folks mount them using self tapping bolts. IE: you drill a small hole, and then let the bolt/screw do the rest of the work for you.
I know a big seller, AJ of AJ's offroad sells his that way. FiFo and I put a pair of AJ's sliders on his rig and I wasn't a big fan of the installation process. They just seemed weak. But the did hold up to a year worth of FiFo's abuse.
In the long run, I would bolt them to the unibody frame rails, and then weld them. When you are making the mounts to the "frame" rails, make them look similar to a chevy "bow tie" so when you weld to the unibody, you aren't weakening it.
Or you can go hardcore, and check out FiFo's short Bus build thread here.
Thanks alot, looked at that build too, lots of ideas, pretty much first set of tube fenders ive seen for an xj- pretty cool, loved how the rears were done
when i put my sliders on i plated the whole side of the unibody under both doors. Then welded to that. I did run my cage through the floor and weld it to the top of the sliders also so i didnt want anything to give