I usually don't have to touch the 6V starters after a 6 to 12 volt conversion. ;-) I have also used 24 volts to power the 12V starters when installing a Corvair engine on an AC Beetle transmission. Those starters spin fast too! 8^)
Lets see you could grind out the inside of the transmission bell housing to install the larger diameter flywheel so you can use larger clutch components and the 12V starter with the special bushing. There are 12V starters from the auto sticks, or Type 4s, that should work with the 12V flywheel that are self-supporting and won't require that special bushing. I guess if you don't want that starter to spin too fast then change out the starter or reduce the power going to it. Won't the 6V Starter MOTOR still spin fast even with a 12V solenoid?
Not sure that I have ever installed a 6V starter drive onto a 12V starter.
Sometimes I do touch/change the wipers to slow them down though! lol