Master cylinder question...

[Oregoon]

So, I'm close to doing the whole rear-end/fivelug/lowering/disc conversion on my '66 Burb.

Naturally, I'm going to lose my single master cylinder in favor of a dual, with a proportioning valve.

Here's the question: My '66 is already power-assisted. Can I just replace the single master cylinder, and if so what year dual do I use?

Or do I have to replace the entire booster and master cylinder assembly with something '72 and up?