Before I put jb weld on my s14 gas tank - Anyone know if it is made from polypropylene? From my understanding, this could possibly leak again when the tanks plastic expands and contracts.

I don't know specifically for the 240SX, but the vast majority of plastic gasoline tanks are HDPE (high-density polyethylene). Polypropylene, when used for fuel, is more likely to be found in a diesel. JB isn't the best stuff to fix either one, you're better off with two or more coats of a liquid epoxy that will creep into the cracks. If the tank is holed and not just cracked, start with JB to fill the hole and then coat liquid over it. In both cases, make sure you anneal any radiating cracks by drilling a tiny hole at the point where the crack stops, this will prevent any further migration. For small cracks use a 1/32" drill, for larger ones use a 3/64" or 1/16" depending how bad it is.