[folivier]

Gotta agree with the others they probably used DOT airline for something that is not DOT regulated. That's what I do, all my spare fittings and airline are DOT so they can be used on the brakes or anywhere else.
I worked for a gas pipeline where typical pressures were 1000psi. This is low pressure stuff. There should be no practical restriction on the top line whether it is 1/4, 3/8, or 1/2" connection.
As far as the tank any fab shop should be able to make you one, maybe even in aluminum. Heck schedule 80 pvc is rated for 167psi!