Well I think I should post the wheel cylinders. Keep in mind these are Left/Right specific with the angled port line.
This is the stock rear wheel cylinders on the '70 2500
The stock rear wheel cylinder for 72 C10 with disc brakes.
Its the correct WC size, but this will not bolt up to the stock lines, however...
The stock rear wheel cylinder for a '70 Fleetwood.
The correct rear wheel cylinder size for use with the 75mm calipers, and the correct line port size of 7/16".