I think once you deviate from the original you can use anything you want. I am pretty careful to specify what hoses and fittings I use on field approved brake systems. For Cubs and Aeroncas I always use the equivalent of original equipment, and I get them approved in block 3.
Interesting story - the STC disc brakes for J3s use copper tubing. The reason is that copper was original, so no FAA hassle. Aluminum tubing would have required engineering approval beyond that required for the calipers, discs, etc.
I don't know what the factory used in the 1970s, but during the 8:00x4 era it was all auto stuff. You go to Aeroquip, presumably, as a minor alteration. Remember, Aeroquip is 600 psi stuff, and auto is 4000 psi.
I better say - opinion. I really caught it for my lousy advice last week - not here - I think a fellow forumite turned me in, or got a contract on me . . .