The '77 YZ-D forks are not the same as the '76 C forks.
I had a '76 YZ-125X with the dual air chamber forks (no springs at all). Those things were also on the '76 YZ250 and 400. The 1976 YZ 125 'C' had conventional spring forks.
For your 'C' forks, I would worry less about the specific cc volume required and more about the oil level. Basically, take the springs out and compress the forks. Fill to about 4" from the top (however much it takes). Put the springs back in and ride. If they are too soft at the end of the travel, add more oil. If they are too stiff and don't use the full travel, take some oil out.
If the oil has been drained and you are replacing with fresh oil, stroke the forks a few times before you measure, so that the oil circulates to where it needs to go.
Oil level has a big impact on stiffness at the end of the travel. Oil weight mainly effects rebound damping. Altering the oil level is an essential tuning technique no matter what the manual says the cc volume should be. I've had some bikes be way off (for example, my '84 KDX200 needed about 2" more oil added over the volume specified in the shop manual)
As mentioned before, I would start with 10w oil. Air caps were common back then, but adding air is discouraged. Use the caps to let air out. Use of air pressure in those forks is a bandaid if you are too heavy for the springs. Instead of air pressure, use spring stiffness, pre-load and oil level and weight to tune the forks.