All my clutch masters are 5/8 bore on my 3 Datsun 320s(62/63/64), all my stock brake masters are 3/4, all my mounting holes on both types are closer to 2 1/4 inches apart center to center.
Them are the same sizes/measurements as the 521 clutch masters, you should be able to use the 521 clutch master, although you likely will have to use the rod on the old master on the new master.
In the photo below the top master is a brake master(3/4), it was removed from my 1962 Datsun U320.
The second master(middle) is marked as coming off my 1962 U320, it is a 5/8 bore, it's obviously a replacement, although the rod looks new, so when I changed it out I used the original rod on the new master.
The bottom clutch master is also a 5/8 bore for the 521, the rods might not be the same length but are interchangeable, the mount holes are all the same distance apart.