High capacity fuel tanks?

on my old setup (Frontier), i had a early 90's nissan pathfinder fuel tank i was going to put in my trucks spare tire location. I believe it held 19-20 extra gallons. cost me about $30 at the junkyard. and an external fuel pump can be had for about $25-$80 depending on brand. I was going to pump into the filler hose of the 1st tank. Would of gave me about 40 gallons total if needed... Would have worked perfectly, but i sold the truck, then sold the tank