64 (Canadian) Valiant Signet (aka Dart GT) front disc brake conversion

Regarding your rear brakes:

Check the rear flex hose. A few years back i helped a coworker get his 73 340 Dart Sport back on the road. We couldn't get any brake fluid at the front calipers. After disassembling the distribution/safety block and all that I finally discovered that both front brake hoses were blocked. The metal ferrules at the end of the hoses had rusted and completely blocked the internal passages.
In reality, since you are into the hydraulics anyway, you should carefully check the rest of the system before hitting the road. I use synthetic DOT 5 fluid in my 60's cars. It doesn't absorb moisture and doesn't harm paint.

P.S. I have a /6 64 Val convertible too. Fun cars.