Use a truck? I was merely pointing out that if there was a leak or other problem, the small tanks would be more than enough to cause a big explosion. The OP suggested that the smaller tanks were safer, which isn't the case. Propane tanks shouldn't be carried in the trunk either. Even dry ice shouldn't be carried in a closed space. If dry ice is in the back seat of a car, all the windows should be open. Welding trucks even have to have vented cabinets and the cylinders have to be capped when being transported. A lot of guys leave the regulators on but they could get a big fine if pulled over.
I've seen cylinders carried all sorts of ways. It's quite a job fitting a 337 cu. ft. oxygen into a Chevette. Every once in a while a customer brings a cylinder in to exchange and forgets the cap. Then they get upset because it's a $10 charge for a cap so just want to exchange the cap with the full cylinder. What they fail to realize is they're carrying a bomb with 2200 PSI! If they bring the cap back the next time, they get their $10 back.