Cruiser, You have started a big project with your 1953
Airstream. You might like to start your "full monte" thread in the Airstream Knowledge base > trailers > and then 53 Cruiser. I think your fellow early 50s enthusiasts will follow your progress in your rebuild thread. I see there is only one Cruiser thread located there. So here is your chance to become the "expert" in Cruisers! And take a look for other threads folks have posted concerning a full frame fabrication.
So I will give you some "free advice" on some of the design parameters for your new frame. First and foremost, the body attachment points are sacred and can't be altered. That goes without saying.
4" frame rails are fine in my view. It somewhat depends on how much weight you plan on adding to the interior. Airstreams were built light. Adding fancy beds, plumbing, TVs, galley appliances all add weight. 4" square channel frame rails would add strength. My 66 C channel frame rails are 3/16" thick material. But the frame rails are 5" C channel. I think this design changed happened in the early 60s.
My 66 Trade Wind cross members are stamped sheet metal 1/8" thick. They have oval slots in them for lightness. There are two heights of my cross members as Airstream provided for "splicing" the 5/8" thick plywood subfloor at certain places along the frame. How you plan on laying your subfloor may affect how you design your cross members. Some folks use "biscuits", and others use tongue and groove. Modern adhesives are very good. So I would probably use tongue and groove exterior grade plywood and not worry about different height cross members. My cross members are generally on 24" centers, but there are a whole lot of them that are at different spacings depending on tanks, entry door, axles, etc.
My frame members are about 58" apart outside to outside. My outriggers are about 15" long. The outriggers form the curve for the belly wraps.
I think you should paint your new frame for rust protection, but not with POR 15. POR products are for "painting over rust" where the paint reacts with the rust to form a rust barrier. New steel won't have any rust. So I should think a good high quality automotive primer and paint would be adequate.
I should think mild steel is adequate for a trailer frame.
Mounting a torsion axle is a good idea. But you have to consider the ride height you want your trailer to end up with. My Airstream has a flat 1/4" plate welded to the frame channel that acts as the mounting plate for the axles.
Others much more experienced than me will give better advice. Here is a recent new frame build thread on a 48 Liner that I found informative. You might enjoy it. Here is a picture of my frame under my 66 Trade Wind.