Master cylinder

The factory valve reduces rear pressure to about 75% of the front;so says the 1969 FSM.I bet a huge percentage of time the rear brakes are doing very little more than keeping the drums shiny. Which is why I threw that Pvalve away. I figured if I had 295s or 325s back there, I might as well use them. Now they do most of the work, for me.I wear out rear shoes probably 2 or3 times as often as the fronts. 295/325s stick pretty good with about 48% rear weight bias. And with 295s back there, they work in the sideways direction too!When I hit the brakes hard, the whole car just kinda gets sucked down.No drama.
With 10" slicks I would try the system without a Pvalve. If the rear locks prematurely, just go down one wheel cylinder size.That's what I would do.
And if it's a track car,or a weekend warrior,the lack of reservoir size to the calipers,IMO, is no big thing.Just top it up more often.
295s have about 10 inches of tread; about the same as what you are gonna run. You can spend a $100 on an aftermarket valve, and spend a bunch of time plumbing it in there, and in the end it will be running wide open, so you might as well just start with wide open.The shake down run will tell you everything you need to know, and changing w/cs is easy/peasy.
My 2 cents