66 valiant disk brake conversion??

The real issue is the limited selection of 14" tires, luckily I think Michelin still has some decent ones for that will fit a '66 Valiant. Most sbp wheels are 14", yes there are some after market 15"'wheels but when you add the expense of brand new wheels you might as well go LBP.
Looks like you already have Rally wheels which are probably 14" so the original Kelsey Hayes systems is a good choice for you and these brakes work very well. Place a wanted add & Im sure someone can help within two weeks. I would buy these calipers with stainless steel pistons Stainless Piston Barracuda, Dart, Demon, Swinger, Scamp, Signet, Valiant 1965-1972 Right Front Caliper
And you should also replaced the single reservoir master cylinder with a dual reservoir version because it's much safer.