The Kelsey-Hayes Disk Brake Swap Thread
I did some research on the master cyl's for my 67 a few years ago, because of the numerous part number I was getting for the same cyl w/disc brake.
From what I have found is there are 3 bore sizes, 15/16, 1" and 1-1/32. As mentioned this will change pedal effort.
The second difference I have found was a bolt on top like the factory and the clip on top like the later models. 10-1326 clip on or 10-1323 bolt on. The noted difference between these two is the 326 is said to be for disc/drum and the 323 is said to be for drum/drum. Both have a 1" bore. The 326 appears is actually just a higher fluid capacity master cyl. I did use the 10-1323 on both of mine as these are like the factory masters that were originally on the car. Had my purpose going to be more then just typical driving - drag racing, road racing etc I would have gone with the high cap cyl. Don't know if this helps.... just some info I'll pass along.