Thread re: Dual Master Cylinder Conversion ('65 w/power brakes)?
Not "a" thread, almost a gazillion.
I would use a newer aluminum MC, but you could use a 1967+ drum-drum MC if you like rusty cast-iron w/ leaky top at higher cost. Newer MC's have 2 bolts, so buy a 2-4 bolt adapter plate ($30 ebay). I did that on my 65 Newport, mounting to its factory booster. The booster has an adjustable rod tip, as I recall. Adjust it so the tip just touches the MC piston as it mates w/ the booster. On my 64 Valiant, I did that for manual brakes. In my 65 Dart, I used a later booster & MC on 74 Dart brackets (avatar). In all 3, I used an MC for a 95-99 Mopar "cloud" car (Breeze, ...) w/ ABS. In the Valiant, the brake pedal wound up in exactly the same place, meaning the piston's hole is at the same depth after adding the adapter plate to the MC. But don't assume, adjust your booster's rod tip.
Re plumbing, search for my post w/ PN's. I kept the distribution block in the front circuit. Note that the top port is for 1/4" tube, so requires an adapter for 3/16" tube. Plumb the rear circuit direct from the MC, but needs an inline adjustable proportioning valve ($30 ebay). Adjust so the fronts skid just before the rears in a wet parking lot. I used the existing rear tubing, using a 3/16" union.
