Tommy, "back in the day" a common solution to the 6 volt problem was an 8 volt battery and an adjustment to the mechanical voltage regulator to support it. I haven't seen any 8 volt batts for sale for years and years now.
Good idea using a switch to make 6 into 12 just for cranking.
I aint got all that fancy learnin' like Prof. Ron got.
Backyard Ernie