Ok, as someone that has actually built his own control arms many times here's what I've got for ya.
#1 - .120 wall is hardly enough for a rollcage, why would you make control arms out of it? NO WAY. Get some good DOM in at least 1.5" x .250 wall. Why skimp on control arms? They break/bend on the trail.....you're f*cked.
#2 - 3/4 heims. NO WAY, the rock buggy guys can get away with that, their rigs weigh about 1/3 of what you're loaded XJ will weigh. When you're building a trail rig, that you want to survive & not leave on the side of the trail for 'dead', you've gotta think BIG. I would use no smaller than a 1" heim, and I recommend JJs. Currie now sells a JJ with a 1" x 14pt grade 8 bolt welded to it. 2.5" thread & a hellofa lot of weld area on the JJ. These work very well.
#3 - Length. This is really the easiest part of designing your own arms. Each XJ out there is running a different tire size, trimming amount, etc etc. So why buy the same arms everyone else is for your CUSTOM project? The way to determine your arm length is this: support your rig on stands, remove all stock arms/coils/TB & etc., position the front axle so that the LCA mounts are inline with the frame end mounts (or drop brackets), bolt a tire onto the axle & position the axle front to back (and rotate for caster) so that the tire will clear the front bumper & turn lock to lock. Measure your distances & build your arms. This is the furthest point that the axle can move forward, thus this is the perfect arm length if you can turn lock to lock without hitting anything. Build the arms with at least .5" each way adjustability & go wheel it.
Remember to set your caster throughout the setup of your axle or you'll be rebuilding your upper arms later!
Good luck, and dont forget that you're not building a go cart, you're building a trail rig that's suppose to be SAFE, STURDY, and get you home after the run. Overkill is usually just enough.