I have a 1965 Dodge Dart 270. They also do not have a brake booster listed for that year as well. I would also like to eventually convert the front to disk brakes so any information on that would be helpful as well.
The last question first - do you want to keep the SBP you have now? If so try and find someone selling a Kelsey Hayes set-up. Lower ball joints would have to be upgraded. If converting to LBP, then 73-76 discs would be the way to go. Change the master cylinder to one for either drum brakes (if keeping drums) or one for disc if doing the disc swap (recommended)
First question - any A Body booster from 67-76 will work. Earlier boosters are something like 7 or 8 inches in diameter; later ones are larger in diameter. A search on here should shows pictures of each. Make sure you get all the brackets that go with it.