OK, I hammered the wheel well for the spare a bit, found out that it had fiberglass over it covering a bit of rot (just going to fiberglass it again until I can get a body guy in there to just cut the thing out and put a flat piece there anyways).
The XJ tank will hang about 3" lower than the stock Eagle tank. I bought new bolts to make new hangers to accommodate the XJ tank which should reach the Eagle straps. After I install it I'm going to piece together a skidplate to protect it, the stock plastic Eagle skid was put on with some fairly decent bolts so I'm going to reuse those holes, surround most of the tank and bolt it to the floor of the rear hatch area for support.