[theastronaut]

I've swapped CPP's drop spindle/disc brake kit on a few 64-66 trucks and none of them needed bigger than 15" wheels. There is a clearance issue with the inner barrel of the factory drum brake wheels- the inner barrel is slanted at a 45* angle and that area will hit the caliper. You can used a bolt on wheel spacer to gain clearance, or use newer 4 nub truck steel wheels that have a stepped inner barrel made for clearing calipers.

Neither of the CPP kits I used needed a booster, they stopped really well with minimal pedal pressure with a 1" bore manual master cylinder. A booster adds complication and has worse pedal feel/feedback, and when you add a booster you have to modify the pedal linkage to take away pedal leverage from the booster/master.... that kind of defeats the purpose of adding a booster. A lot of booster/master cylinder combos are 1.25" bore, again reducing the amount of "leverage" your foot has on the brakes so that the pedal isn't too easy/soft. Try it without a booster first.

Make sure to use a master cylinder that is made for discs, the stock master cylinder has a build in residual valve that will cause discs to drag.