I am in the "behind the seat is the best location" camp. If fact, my truck had a tank mounted under the bed when I got it and I am just finishing up relocating it back behind the seat. I'm VERY thankful that the guy who built my truck didn't eliminate the original filler hole and also didn't add a filler anywhere else. It was located behind the tail-light (fleetside) so putting it back proper did not require any major repairs. Something to consider, GM (as an example) put the tank behind the seat for decades right up to and including 1972. All that time, they had tanks mounted between the frame behind the axle in the suburbans and deliveries. If that location was decisively safer, wouldn't GM have just used the same tank in ALL truck applications?. I've seem many pre '73 trucks that have been smashed from every angle from fender benders to almost unrecognisable but can't recall one that suffered a fire as a result.
