65 Valiant Signet convertible project; where to start?

With a V-8 you probably have 10" drums. I would just order a 1967 A-body master cylinder and plumb it to convert to a dual system. 1967 is the first year dual braking circuits became law. Since your distributor is messed up, now would be the perfect time to gather the parts for a factory electronic conversion.

You have an interesting project going. I just started on a 64 Val Vert myself. My immediate plans are to get it running and stopping first. (been off the road since 82) My immediate goal is to start driving it by summer. I just dropped the /6 block, crank, rods off at a machine shop about 1-1/2 weeks ago. It needs to be bored. Interior needs work but I can live with that. Exterior is OK and it has a patina I went to enjoy for awhile. I guess they all rust behind the rear wheel opening. Once winter comes again I'll do more. Since you are now in Florida maybe weather won't be a problem for you but I typically drive my junk in the summer and collect parts for the winter projects. That's my plan but you have to come up with your own. Good luck.