Hi from snowy Colorado: It might be a bit early to consider axles. I can tell you that Airstream went with these new "Dura Torque" axles back in '62 or so. They eliminated the problematic leaf springs and shackles. They are also "independent" suspension greatly improving the handling of the trailer.
I read some Liner owners have upgraded to the newer style axle.
I would recommend upgrading to the dura torque style axle. It would come with 12v
electric brakes on 10" by 2 1/2 wide drums, and new bearings. This item will be one of the last things you assemble to your trailer.
I should think you will want to first do a thorough assessment of your 70 year old Liner. You need to consider the desired use of this old Airstream. If you desire an "aluminum tent" used for sleeping out of the elements, that is one thing. If you desire a fully contained travel trailer with water, bath, galley, 12v
system, 120v system, heat and the like, that is something else.
I'd jack it up about 20" off the ground and then drop the axle, and drop the belly pan to expose the infamous "pipe frame". You need to check it for rust damage. And you need to study the possibility of replacing the frame as many Liner owners have done. If this is desired, then you can design in the correct specifications for a new axle.
I encourage you to keep reading in Air Forums about Liner build projects and decide your end goal.