The key problem is that B&W film, specially during WWII, was "color blind", or properly speaking, not very sensible to certain wavelengths (colors). Just search for orthochromatic (the usual one, which gave almost black for red colors, as it was red-blind. Remember the darkrooms with red bulbs?), and panchromatic films, the better one at the time of WWII.
Nowadays you get a much more balanced image, regarding sensitivity to colors, from a electronic sensor, and that´s fundamentally different. Whatever usual conversion you do, will produce "too many" gray levels, I mean too many because one of the prominent features of old pictures of lower quality film is the high contrast, or not many different gray levels.
The good news is that there are software tools that imitate this film effect, I remember years ago to have seen some "filters" or "plugins" for photoshop. I imagine it has to be relatively simple to simulate a color response diagram from any film. You may try to search the net for that kind of plugins... I am disconnected from this world since years, I can´t stretch my time more...