My 2 cents, I put in an 8 volt battery many years ago and for me it has worked very well. It did require me to adjust the voltage regulator. Starter spins like twice as fast and I did not have to change any bulbs or gauges.
This was advertised for hard to start farm tractors at the farm and fleet store maybe 20 years ago and had instructions for adjusting the regulator. (one screw, but I don't remember which one).
