It's best to get the master cylinder up as high as possible, while still clearing the bottom of the K member. If you have it too low the lever on the brake pedal ends up a bit long and you lose leverage on the pedal. there was a thread on this recently, but it may have been on the hamb. I'll see if I can find it.
Edit: try this. page 7 has some master cylinder discussion.
https://www.jalopyjournal.com/forum/...-build.1157996
Mart.