Ah, the Pandora's box of radial vs bias ply tires. Technically, KF rims were not of the sealed variety until at least 1951 which means rims prior to that should use tubes. I have tubeless bias ply tires on my 47k and 50K and have never had an issue with leaks.
My 53K had radials on it and I had no issues with handling or roadability, although I'm sure there will be those who say otherwise.
You will hear from many who say you should never mount radials on rims intended for bias ply tires due to how they flex. I have never experienced an issue and do not know anyone who has.
For sizing, your car originally had 7.10x15 bias ply tires which would translate to 215/75R15 as a radial.