First off IDK Jack about XJs so I can't comment alot about that.
But Id like to differ on your wood floor. You said you have $326 in it and said it was way way cheaper than concrete.
10'x20'x.33(4"depth)/27=2.44 yds of concrete
2.5 yds x $120per yd=$300
$300+$50 delivery =$350
$350+ $20 in form boards=$370
Screed it out, borrow a hand trowel and smooth it out, and you have a concrete slab that will last forever.
You saved a whole $44
I just hope you finish the jeep before the wood boards rot into the ground.
Lots of good info is being given to you freely by some very knowledgeable guys. Don't let that go to waste.
Just like the wooden floor (as w/ the jeep) keep in mind cheaper isn't always better. Especially when it comes to safety and longevity.
Good luck with the build