GM used 2 types of timing marks. One was at the 2 o'clock position.
The later they went to a 12 o'clock position. Since the balancers interchange, in some cases the builder will install a 12 o'clock timing cover
and a 2 o'clock balancer. When this happens you can't time it as the line won't match the timing tab. The easy fix in that case would be just to go buy a bolt on timing tab that bolts on at the 2 o'clock position.
Now if it were vice versa and a 2 o'clock cover was used and a 12 o'clock balancer then you would have to go online and find the formula on how to find the 2 o'clock position on the balancer. Or, you could just do as I would and purchase a proform balancer cover that has timing marks from zero to sixty degrees. It don't matter what balancer you have when you use the proform cover, because it bolts on to the balancer and has a mark on it so you can line it up with the keyway on the crank. When GM made the change in timing marks they left the keyway in the same place and simply cut the timing mark on a different location on the balancer. The Proform cover places it at the 2 o'clock position period. These covers come in sizes for 6 and 8 inch balancers and well as several different colors. When I do a build I get the color and size I want and use a bolt on 2 o'clock timing tab.