Looking for a firm brake pedal

Sounds to me like you are doing a lot of guessing.

The real answer is, you need to find out what you are starting with. If you are absolutely certain that it has the original cylinder, look it up and find out what the bore is, and if you can't then pull it off far enough that you can stick a caliper in the back and measure it.

Or if you know the year/ model which the 10 inchers came off of, buy a master to fit that application