There is no "safe" place for a fuel tank. Just safer. Between the drive shaft and frame would likely be the "safest" but there isn't much room there especially on a SWB. I've seen many pre 1973 trucks smashed up over the years and don't recall ever seeing one that caught fire. I'm certain that it has happened and when it has, it wouldn't be good for the occupants, but I think the chances are better for a fire in a rear mounted tank (behind the axle) in a rear-ender. In the videos I've seen, the tanks don't "explode". They rupture, and then, as the crumpled vehicle is push forward some metal scraped the road causing sparks igniting the leaking fuel. For a seat tank to rupture, it would most likely be if the truck got T-boned and, in such a case, the truck won't get pushed as far and it would be less likely (I think) that metal would be scraping the road. Of course, there are other sources for ignition in a wreck. I don't know what the 'right' answer is but I do feel that, should I get in a wreck, I'm more likely to be rear ended than T-boned.
