Boosters are hard to get. Good luck finding one that fits at any auto parts. I lucked out and grabbed a new one (Bendix type) for my 65 Newport on rockauto for $70 (clearance). Usually, you must try somewhere like "Booster Dewey" and pay $300. I have not seen rebuild kits. If desperate, you could drill holes to fit another booster to your "booster plate". I had a Midland-Ross booster from a 74 Dart that looked just like the one (replaced once) on my 65 Newport, but the bolt pattern on firewall side was different. I drilled 3 holes and made it work, but one hole was in a rippled area. I later got the correct booster (above) and sold the Dart one. I also tried fixing my original M-R booster. I bought neoprene sheet (ebay) and made a new rubber diaphragm (mine had a hole right @ vac port, though I had used a fuel vapor filter). It works when tested off-car w/ vacuum, but not confident the rubber will last long (haven't installed). Another data point, the booster on a 95-99 Breeze or LH cars has bolts real close to a Dart, since I mounted one on Dart stand-off brackets (photo above) to use on my 65 Dart w/ just a little rat-tail filing. Indeed, they are closer together in width so might have fit better on my Newport's plate. If willing to use A-body stand-off brackets on your C, you can buy a booster w/ MC & brackets for ~$170 on ebay. It uses some GM parts. That would help insure you clear after-market parts (valve cover, headers).
Once you have a booster, the MC is easy. Any 4-bolt Mopar MC will bolt up. If you dont like rust and weight, use any 2-bolt MC w/ 2-4 bolt adapter ($30 ebay). Many use an 80's Dodge truck MC. I used a 95-99 Breeze w/ ABS MC on my 65 Newport's booster. Disk/drum MC's have a larger reservoir for the fronts (aft of MC) or later MC's have a shared reservoir. Bore size is preference. 1-1/8"D (above) sounds large, but depends on the caliper too. Most A-body guys use 15/16"D for an easier pedal (at expense of more travel). My C-body has a 7/8"D and the pedal barely travels. With the booster it stops "authoritatively" w/ drum brakes, but would be about right w/ front disks. Insure you install a proportioning valve in-line w/ rear drums or the rear will lock-up first and cause the car to spin out. Adjustable ones are just <$30 on ebay.
ccasion14:
The dual master cylinder is really easy to plumb and a huge increase in safety. I agree on the front disc rear drum school of thought. Purchase a premium 11 x 2 1/2 shoe and make sure the drums are square and you should be good. The front disc conversion is easily done with all Mopar components. See the Disc-o Tech article every where on the internet for the complete how to. Also remember that you can decrease pedal pressure needed by reducing the bore of your rear wheel cylinders once you change out the MC. If you have clearance issues with a different booster use an "A" body 6 cyl spacer bracket to move the booster away from the fire wall, bolt pattern is the same.
