I don’t have a lot of room between the rails, it’s a 1940 and the frame rails aren’t as far apart as later trucks, plus I’ve got the torque tube and exhaust. I think the under seat location is worse than outside the frame because I’m basically sitting on the gas tank and an ignition source (battery under floor).
The tank I’m looking at has a Metal plate on the outboard side of it. I could put some foam between the two. It would stick out 20 inches from the frame. The bed is 28 inches out from the frame.
