Technically, no you don't need a new master. Your original one will stop the car just fine. However, a disc brake master will have a larger reservoir for the disc brakes.(The disc brake calipers use more fluid then drum brake wheel cylinders) It's a good idea to use a disc brake master cylinder.
You also will need a prop valve. The valve regulates the fluid front to back. Originally, your car would have come with a distribution block but not a prop valve. If you had 4-wheel disc brakes, then you don't need the prop valve.(your original distribution block would work for 4-wheel discs)
There have been issues with the reproduction prop valves. Since you aren't worried about looking stock, I'd look into an aftermarket adjustable prop valve.
I believe your original brake pushrod will still work with a new master cylinder. You will probably have to get a new O-ring though. The O-ring on the end of the pushrod keeps the pushrod locked into the back of the master cylinder.
