I replaced my 15 with the OEM 21 gallon tank (not the aftermarket 21 gallon tank) to keep my ground clearance. The 21 gallon aftermarket hangs down about 4 inches lower than the 15 gallon. I also picked up a 4x Doctor skid plate for it that is incredible. The 21 OEM units are a bit harder to find but they are out there. (They fill the entire area between the leaf springs/fram rails rather than hang down further to get the capacity). Hope this helps. Either way poly is a the best way to go, if you have a good skid plate.
1984 CJ7, 258, MPI, 4.0 head, Borla, 700r4, amc20 (detroit), Dana 30 (OX), 4.56 gears, Warn Hubs, Dana 300 with 4:1, 4.5 RE lift with 1" body, Bilsteins, dual battery, PS, PB, full cage, two dogs in the back....