When you do it, you could also rust proof the seams after completion with a rust restorer/preventative like por-15. If you weld them just take your time, if you try to run a long bead on sheet metal, it will warp your new pans and possibly your rockers. Solid tacks and short beads are the best way to attach them, from both sides if possible. Or, you can pre-drill your new pans as if you were going to use rivets, but tack in the pre drilled holes, same equivalent as spot welding. Make sure that you seal it after you've finished, whichever you decide to do.
If there's a will, usually there's a way, but then you get to that bolt that you still can't get to with two extensions, two universal sockets, and a contorsionist midget trying to help you.
1999 XJ Four door, mostly stock for now, but I am in the process of building a custom plate bumper.