Disc brakes require a booster as the master for the drum brakes didn't require as much pressure because of the way drum brakes are designed. The entire process of mounting a master on the firewall is a major mod and you are already asking the correct questions as there are many challenges.
Properly adjusted and maintained drum brakes work perfectly so there's no "need" for disc brakes. Hot topic, Im sure someone will tell me I'm wrong or old fashioned, but I can assure you that what I tell you is true.
My suggestion is you get the drum brakes in top shape, learn how they work, how well they work and keep your money for things you should pay more attention to. For example, replacing the wiring harness would be easy, and give more piece of mind than any disc brake conversion. There is a plug and play harness on the market for $300 that you could easily install in an afternoon. 60 year old wiring can burn that baby to the ground with no warning!