To keep the 8-Volt battery properly charged, the voltage regulator should be "tweaked", and the extra voltage will shorten breaker point and coil life. Do you know how to set the regulator up a bit, or is there in old-time auto-electric shop nearby that would be familiar with it? (An "8-Volt" battery will charge at about 9.8 Volts.)
That being said, lots of old tractors are running around, more or less succesfully with 8-Volt batteries.
If it were mine, I'd clean and tighten all connections, making sure the battery cables are of adequate size, and keep it as-original, 6-Volt.
Check to be sure the generator is putting out about 7.4 Volts, after it's run a while, revved a bit above idle.
Or, if you really feel the need for lots of cranking speed, make the changes necessary, and convert it over to 12-Volts.