Hi Nick,
Are you permitted to run the later type 260Z rear wheel cylinders in your class? I ask because the later (1976 onwards) 260Z came with a dual piston wheel cylinder arrangement which is infinitely better than the sliding piston arrangement found in earlier Z rear drums.
Without having checked it myself I would assume that the sliding single piston arrangement found on 240Zs would be prone to wearing one side of the cylinder wall (and also potentially cause uneven brake shoe wear) because as the wheel cylinder lever acts upon the piston and slides it up the bore, the leading brake shoe will be trying to pull the piston downwards and wearing one side of the cylinder prematurely.
To convert to the later type you would need new shoes, wheel cylinders, adjuster mechanism and backing plates, the drums themselves are the same.
They look like this: