Its time to stretch the wheel base on my CJ-5 and this means removing the fuel tank.

My question is there a preference of stainless steel vs aluminum. The tank will be a low profile with about the entire rear floor of the tub being covered. The tank will have a sump and baffles. The engine is fuel injected and I use an in tank pump.

Do any of the rock crawling sanctioning bodies have any rules that would prohibit aluminum?

Unless you get some sort of "deal" on aluminum, there´s no good reason to prefer it over stainless. There´s very little weight difference, at least as a percentage of the filled unit. And aluminum os a LOT weaker than stainless in comparable thicknesses...

GO STAINLESS!!

HTH