Do you have brakes that came on the bike? If you do, you could start by measuring that.
This tells you how to measure reach:
https://www.sheldonbrown.com/calipers.html
Old bikes have a lot of clearance. I'm getting 65 mm up front and 75 mm in the rear with 27 inch wheels on my '68 Paramount. The brakes on mine which are original to the bike are old school Weinmann 999s with a heck of a lot of reach.
