75 f100 rear disc

I just bought a 75 f100 project and the truck currently has disc/ drums. I found a kit on speedway motors to convert the rear of my 9in to disc. It says I will need to run a different prop valve for disc/disc and run a larger master cylinder to get the right ratio back. I’ve already found the prop valve. I was just wondering if anybody knows what other master cylinder that will work with my truck to give me the right ratio.