Loadmaster would be AD truck right, or late 47? does it look like my trucks, or older?
Original transmission will be fine with a 350 in front, the same transmission was used factory for many years with the V8, and is well respected by the 4X4 guys.
My 1 ton that had the Dana 70HD was just a test for installing them in a 2 ton. I had the 1 ton, and was not ready to start on a 2 ton quite yet, so I built the axles as the first step in my 2 ton project, and tried them out for 10,000 or so miles in the 1 ton. Both front and rear axles I had in the 1 ton are now under my 2 ton.
My gearbox is from a 78 IH Scout II, quite different from the popular Chevy 4X4 box, other than being made by the same company, Saginaw. My setup won't work with the original steering geometry. With the Scout box The drag link has to attach to the front side of the right wheel.
I would not worry so much about the axle shafts breaking, not much you can do but put another equally old one in. If one does break there is no real danger, just pull over, not like a wheel will fall off.
If you had new ones made I don't think it would be worth the money, as the old ones may never break, and used ones are cheap.
Grigg