66 Dart master cylinder replacement

I would get a modern aluminum master cylinder. Mopar Performance sells one, but you can do much cheaper. You can buy an adapter block ~$70 so the 2 bolt holes can be used with the 4-studs on your firewall. I plan to try just drilling 2 holes for an MC from a Breeze (no booster) on my 64 Dart.

To get power brakes (not essential with drums), you need the stand-off brackets w/ lever from any A-body. You can mount just about any booster and MC to the brackets. I used one from a 95 Breeze on my 65 Dart. I just needed to rat-file file the bracket holes slightly. Or you could buy an after-market set for ~$250. You don't need a proportioning valve with drum brakes. I used one to allow for future front disks. The one shown is ~$25 on ebay.

I suspect you can't use the 10" wheel cylinders in 9" drum brakes. I have both (65 Dart & 64 Valiant) and I recall the cylinders and rods are quite different. Many wheel cylinders are <$5 on rockauto, so I usually just replace them, but that is when rebuilding the whole suspension. I recall that in my 10" brakes I couldn't mount the cylinder without unbolting the backing plate, at least with my wrenches, so might be easier to hone the cylinder in place and install new parts (~$2). You should pull a wheel first and see which brakes you have. You should clean and repack the bearings, which requires a new hub seal, so you need to know. Most V-8's had 10" brakes and slants 9". My 69 Slant had factory air and 9" brakes.