It really depends on how you want to proceed. You can find a Chevvy with a similar weight/brakes and take that master cylinder and use it, or use its rebuild kit. There really isn't much difference in brakes as long as you stay with the same weight class and brake type (vacuum booter, hydrovac, air over hydraulic).
My CCKW has has no less than 4 different hydrovacs over its lifetime.... the latest one (Bendix Series 4) is from the 50's and bolts right up. And this is for a 12,000lb truck. My Chevvy (a 43) is using a late 50's hydrovac also. None of the internal parts are the same, but it puts out the same PSI to the lines.
Have you tried this site:
http://6066gmcclub.com/gmcguy/index.php
