As a very general rule of thumb, "points closed" time needs to be about 2/3 of the total angle between high points on the distributor cam. For a V8 engine with 45 degrees between cam lobes, the dwell usually maxes out at around 30 degrees. Since a six has 60 degrees between lobes, theoretically, the dwell could be as much as 40 degrees. For some reason, the later stovebolt distributors dropped the dwell from 39 degrees to the low 30's. I believe that corresponded with the switch to 12 volts, but that's just a personal assumption, and all of us know what "assume" means!
Jerry
