What is the pressure setting of your radiator cap?
A variety pf types of "anti-freezing" solutions were used in 1946 (methanol, denatured alcohol, ethylene glycol, and glycerine) and some of them were corrosive. Are you using any type of anti-freeze? Straight water would be more corrosive than a modern anti-freeze (which have anti-corrosive compounds added).
It is very hard to remove rust and sediments from an installed engine/block. Drain and flush the system as suggested by Old Dollar. Then add a modern anti-freeze (usually 50-50 with water). I try to find anti-freeze that does not contain silicates (it also is usually less expensive).
