Too many years ago I had a 50 Oldsmobile. Went to 8volt battery. The guy at the battery shop tweaked, (bent a tab to increase spring tension) as I recall. It worked really well for as long as I had the car.
I think if you can take the cover off and get under the dash with a small wood stick and close/open some points you may have an ah ha moment. But then again you could cause some problems.
If you take the cover off, try connecting and disconnecting the battery while watching the regulator. ... I think something is getting switched on by the 8 volts. Just might be a tweak to keep that from happening till say 9 volts, which would be when the gen is spinning up good. hth