Raybestos RC4176 Caliper sourcing

RT Dude said
... All I need now is a correct disk brake proportioning valve for a 68 Dart and I can start the conversion.

In 1967, the prop valve was a separate piece and quite rare today, though I recall seeing it on a website (Inline Brakes?). I don't know about 1968. It might have moved into the "combination valve". Try Google Images.