Would this brake conversion work?

I would keep the stock wheel cylinder sizes, but I like the adjustable prop valves better than the stock setup. Just run your new brake lines directly to the front brakes and put the prop valve inline to the rears. Done it on a couple cars and it works great.

A proportioning valve is for a disc brake system. He does not and will not need one. All he needs to run the dual master cylinder setup is the dual master cylinder, a new distribution block and he adds two lines. That's it.