Safety isnt a good excuse. You have a much higher risk of an accident by putting it under the bed then you would with having it behind the seat.
Between the frame rails under the bed, you have a larger chance to damage it if in an accident, whether being a side impact or rear impact, due to the height and placement of another car or trucks front end. Behind the seat its lifted above where everything but a large vehicle, height wise, could effect it. The exception being a roll over accident in which case you probably will not fair well regardless.
Its hard to argue with the fact that they put them under or behind the seat up until the 1970s. I cannot recall any accidents where having the tank behind the seat was detrimental to the driver.
Just my opinion.
Jeff
