ok here is the scoop on the "15" gallon tank. The 15 and the 20 gallon plastic tanks are exactly the same tanks. The difference is that there is a hose that takes up the difference in gallons. There is a writeup on how to switch from a 15 to 20 gallon tank without changing anything but this hose.
1998 ZJ 5.2 Auto
1989 Wrangler Black Bone Stock 2.5l 5spd
/l , [____],