I don't think having the tank behind the seat is any more dangerous than having it on the side of the frame or at the back of the frame . They're more prone to getting hit there than in the cab. Just as long as all of the lines are fresh and there are no fumes in the cab. But, on my '64, I'm relocating my tank to gain legroom and stereo component room.
Scott

Scott Ward

2 1948 1.5-ton Loadmasters
The red one and The snow pusher
1 1950 3100
1 1955.1 Chevy 6500
1 1954 Chevy 6500 2-Ton
1 1955 1st Series COE 5700
1 1963 K20 (454)
1 1964 C10 (350)
1 1951 1.5-ton Dump Truck
1953 and a 1956 Ford F800

Raising a teenager is like trying to nail Jello to a tree!