The reason I asked why, was that I really cant imagine a safer place for it then behind your seat.
I think it far safer there because its elevated above where the typical vehicles bumper would hit you in a collision. Under the bed you have an easier chance of having someone rear end you and tag the tank or even t-bone you it could still get squished in between the frame rails. Heck they put them there until the 70's without having issues. I certainly cant recall anyone I know going up in flames because of it.
Thats a pretty good track record. Also if the accident is that bad that it squishes behind the seat your probably not gonna be in to good of shape as it is.
The only bad thing I can see is if you get in a roll over accident would be having the filler nozzle scrapping the ground as you roll, but even then what are the chances youll be fine??? Not trying to dissuade anyone or put anyone down for relocating it, just trying to show a different perspective.
I think (I hope) most of us that have this hobby and enjoy driving our old classics pay closer attention to whats going on around them while driving. I know I do, my trucks dont go that fast and are pretty much manual everything. Not like todays vehicles that drive themselves...
Good luck however you choose. Jeff