How well-maintained are your original drum brakes? Have you rebuilt the system lately? Are they well adjusted? Do you have any fluid leaks (wheel cylinders are notorious for leaks)?
My 63 C20 has the original drum system. Although it isn't used for mountain driving, those old brakes have plenty of stop if they are well-kept. It may be worth revisiting them before pouring a lot of money into a conversion.
Here is a tech tip on the subject that may help, if your drums really are beyond help . . .
Disc Brake Upgrade.
