I don't think splitting the brakes diagonally is a good idea. The trick in splitting to a dual master is the balance of front versus rear brakes, and a diagonal split means you have that challenge twice.
I would investigate the master cylinders used on '68 to '71 trucks. The dual master became a Federal requirement during that period and the switch to disk brakes began in '71. I'm sure you'll find that something in that period at least gives you the right specs.
A residual valve serves two purposes.
One is to keep brake fluid from all running back into the reservoir and that only matters if it is below the wheel cylinders. That would not be the case in your '67 C50.
The second purpose is to keep the brakes ready to apply. For drums, that's keep some pressure on the springs, for disks its keeping the pads close to the rotor.
The issue is to hold the shoes against the brake springs you need about 10 pounds. That much pressure will actually apply disk brakes, so they need much less pressure, usually 2 pounds.
Your application should have 10 pound valves in both the front and rear circuit, though it may not be critical and the residual valves may in built into any master cylinder you buy.
Getting the balance between front and rear right can be handled with a porportioning valve. That valve will allow you to limit the pressure to the rear to prevent the rears from locking up before the fronts.
If you can choose a master with bores the same as the stock one that is probably not an issue either.
My experience with the cheep rebuilt master cylinders is that they all leak. Everyone I've ever bought leaks from the rear portion into the front, requiring frequent refilling and bleeding of the rear brakes. I'll never buy a cheap rebuilt master again.
