66 Plymouth Valiant help!

You will need to see what brakes you have on the front. It is possible you have small bolt pattern disc's. The early cars had an option for Kelsey-Hayes discs. These had 4 piston calipers, and the small bolt pattern. Parts are pretty hard to find for these.

Calipers and rotors seem readily available as yet. Maybe a few years ago they were a little harder to get but not now.