i welded square tube that i cut in half onto the uniframe. some people even just use angle iron on the outside of the rail. the uniframe has that crappy coating and is really thin so it sucks to weld to imo.
if you support the uniframe rails and tie a cage into it, you should be stout. i hated my uniframe so i cut all of it off behind the front seats and tubed it.
if your jeep is to become an offroad only rig, you may want to look into this. a good rig to study is cody's zj (the unibuggy) as he plated the rails and tied his suspension links into the crossmember.
there is also a thread on pirate about "beefing the xj unibody" that contains some useful info. if you search you should easily find it.