First question is why do you want to keep the stock axle on your 1939?
Use an axle from a 1941-1955.1 1/2 ton. This will give you bigger kingpins. If you look specifically for a 1953-1955 front axle, you will have the added benefit of the bendix brakes, which is a dramatic improvement over the huck brakes your 39 has.
The 41-55.1 [truck] axles are a direct bolt in for your 1939 axle. There are many aftermarket kits available to bolt disc brakes to the 1941-1955 axles. These kits might not work with your stock wheels. My $0.02 is to get the best brakes that are available. You will never know when a child might run out into the middle of the street. Stock stovebolts stop about as quickly as "Bambi on ice"
