Welcome to the site! You have found a great resource for information regarding overland travel (non-rock crawling stuff), and outfitting your jeep for said use.
A 30 gallon tank is going to be pretty big for any wrangler (LJ or not). There simply isn't much room underneath them as so much *stuff* already hangs so far below the frame. All of the aftermarket fuel tanks I have seen are very expensive for what you get. IIRC, Currie charges around $1500 for their 24 gallon TJ tank (which should be the same as your LJ tank---but I will admit I have not crawled around under a LJ to check). You might consider adding one or two smaller auxillary tanks instead. There is usually some room right in front of the rear axle inbetween the frame rails. You will want to leave ample room for the driveshaft as the suspension compresses, and be sure to add some protection from the rocks (even though you are not planning on rock crawling---wranglers sit pretty low).
"Life is meant to be enjoyed, not endured" ~someone