Phil:
In most cases, it is illegal to change the type of braking system that a particular car came with. It seems to depend upon the organization you run with. Some of the clubs out west seem to follow somewhat of an "anything goes" policy while others, like the club I run with, want the cars to remain pretty well true to their roots (or Rootes...maybe...).
As far as my group, CVAR, is concerned, I'm not allowed to switch to disc brakes in the rear. Consequently, I'm still running rear drums. I am, however, running a '74 Super Beetle master cylinder, mainly to acheive a dual-circuit braking system for safety. My car is a Series I but I run both a Series I master cylinder reservoir and a Series II reservoir to feed both circuits of the brake master. I like doing things like that and using stock Sunbeam parts; it may not be exactly original but, to the untrained eye, it sure looks like it is. In my opinion, by the way, switching over to disc brakes in the rear would be a complete waste of money. During my involvement in vintage racing with my Alpine, I can say with authority that you will be able to out-brake just about everything on the track anyway. As a matter of fact, almost all of my passing is done under braking. You will find that Alpines are very stable cars on the track.
As far as setting up a stock Alpine for racing, as I told you in an earlier e-mail, you will most assuredly be miles ahead if you purchase a car that's already set up for racing. The costs to prep a street car mount very quickly and, without a doubt, it's always cheaper to purchase someone else's work. I know from experience that you will end up with much more invested than the car is worth.
Best of luck,