I know you said truck. Truck can mean a L200 4dr long wheelbase in black w/ chrome, any 4x4, disco, RR, shogun right up to 13.5T.
A truck may well have a trunk/boot with split-rear seats - or a flimsy piece of thin plywood. A truck may well have a pickup bed, but still have a rear window through which objects can be launched. Commercial pickups tend to have wire mesh on the rear of the bed frame, chrome style-bar pickups tend not to.
If carrying a heavy object in a vehicle exposes you to the risk of it hitting you from behind in an accident, restrain it. Not rocket science. Ok, it could become one if a 4800psi aluminium scuba tank, that could be entertaining.
If impractical to mount vertically, not an acetylene cylinder, then restrain horizontally. I recall doing that with a large concrete drain across both rear seats - 200lb, used both safety belts. Safer than having it longitudinal with the split seats down so a rear impact propelled it the extra few inches into my spine or poorly tied down so a frontal impact did the same.