66 Dart master cylinder replacement
That is a v8 in his post. From his carlist on his tagline I would say it is a early a.
Yes, a 1965 Dart w/ 273, standard thick motor mounts. I forgot the diameter of the Breeze booster, but recall ~1/4"D less than a 73 Dart Midland-Ross booster I have (currently on my 65 Newport). I have only ~1/4" clearance to my Wiend valve cover (taller than stock?). My trans mount might be sunk and the engine may come up a bit when I replace it (good since my Pitmann stud barely clears the mini-starter). If my booster then hits the valve cover, I have a smaller one I can swap (for 4 bolt MC, but can adapt). BTW, I managed to repair the original Midland-Ross booster in my Newport by cutting a ring of neoprene rubber for use as a diaphragm. I found no place to buy replacement diaphragms. Haven't put on the car yet, but tests OK w/ engine vacuum.
Re comments on wheel cylinders and proportioning valves, not smart to swap parts willy-nilly. Too big a wheel cylinder can make the rears lock-up and the rear spin out on a turn. 4 drum cars use no proportioning valve as far as I know, with the ratios designed into the wheel cylinders used. That part in ~68+ drum cars probably contains just a "pressure imbalance" warning switch. In earlier single-tube MC's like my 65 Dart, it is just an open distribution "T". In a disk/drum car (73+), the proportioning valve decreases pressure to the rear drums (plus warning switch). They never decrease front pressure. While disk calipers have a real big piston, they need that because disk brakes don't have the "self-generating" effect of drums where the shoes get jammed into the drum by the braking force. An after-market adjustable proportioning valve like in my photo might be best ($25). Adjust on a wet road until the rears lock up first, then back off.
For this simple upgrade from single to dual MC, nothing is trivial. In my 65, the single tube is 1/4", while I think all duals are 3/16". Therefore, you must install a 1/4 male to 3/16 female adapter in the top port of the T (or use a 3-port 3/16 T). Plug the port that went to the rears and use an in-line coupler to the rear. You can get these parts are Autozone in the bubble packs. Count on some rusty tubes breaking, so have a double-flare tool ready (not a cheapy). Note that even with 3/16" tubing, different size nuts are often used so one can't mis-wire at the MC. Always inspect the tube size of inverted flare fittings. Just because the nut fits doesn't mean it will seal. I removed 2 manual MC rods recently. In both, I had to hold the MC in a vise, put a socket extension thru the rod hole and beat real hard with a hammer until it popped out. An O-ring secures the tip in the MC piston, and gets hard. It might be a square ring (never found enough pieces left to tell). Look in the faucet seal section of the hardware store for something to work. I expect the rubber just keeps the MC from rattling. The rod can't come out of the piston once assembled in the car.