Switching to front disc brake questions

You should definitely install a dual reservoir master cylinder as part of that upgrade, especially for a daily driver. And you’ll either need an adjustable valve for the rear brakes or a brake proportioning valve from a disk brake car. Both will require you to bend up some new brake lines.

You shouldn’t just go junkyard diving for the master cylinder. Reason being is that you need a master cylinder with the correct bore and stroke. At the very least you should buy a master cylinder for a disk brake equipped A-body. I run 15/16” bore master cylinders from DoctorDiff on my cars, they’re a little more expensive though. Either way you want to pay attention to the bore and stroke, it’s not a situation where you can just grab a random master cylinder.

I don’t know why anybody makes a disk brake kit for those 9” drum spindles. I can’t imagine spending that kind of money for a disk upgrade and keeping those 9” drum spindles with their inferior lower ball joint design. There have been failures of those undersized lower ball joint bolts.