I'd definitely recommend a bias valve if you're using discs at the back or you'll find the rear is over-braked. Really the standard drums are more than up to the job of giving you enough braking at the back, there's just no weight there to take advantage of anything more. I use a bias lever in mine, I have 200mm drums on since I replaced all the drums and stuff and the bigger parts were no more expensive.
I use a 22mm master cylinder.
I'm really happy with the brakes on my g40, normally when you get in a g40 after driving a modern daily all the time you think "shit the brakes hardly work!" but in mine it makes the brakes on my daily drivers feel crap.