Clutch pack clearances are very important, but main line fluid pressure is just as important. On your C-4, we at Mike’s Transmission would set up the direct pack at .010 per clutch, and .005 in the forward clutch pack. Depending on the torque output of the engine, etc, you will need your pressure to be about 100 – 220 PSI. Less with more power, and more pressure with more power depending on the car setup.
You can also email Mike Stewart at firstname.lastname@example.org