66 valiant disk brake conversion??

SBP is not limited to 14" wheels. There have been links to a company who sells 17" alloy rims for SBP, and fairly reasonable ~$170 ea. Re 14" tires, it isn't a problem for a driver. My 1980's M-B use 14" and I find a wide selection. I have Yokohama Avid Touring S on one car, which are slightly up-scale. Probably limited selection for racing type tires, but I wouldn't pay that much anyway.

Check ebay for a booster/MC w/ brackets. I have seen for <$140. Appears based on a common after-market GM booster, but all interfaces match. Yellow chromate coating. But, many people here are OK using a manual MC w/ front disks. Just get the smallest bore you can find. A 2-4 adapter plate ($30 ebay) lets you use many newer aluminum MC's. I have one w/ 7/8" ID bore on my 64 Valiant (95-99, Breeze, Cirrus, Sratus w/ ABS).