I had a 67 BMW 1800 that was six volt. I put a 2002 12volt motor in it, but had tons of problems with things like the windshield wipers going spastic and the guage lights getting too bright! I tried putting resisters in line to slow the wipers down, but didn't really help that much! It's a pain to do unless you can find the right aftermarket parts that are made for 12 volts. But like everyone has said, 12 is the way to go. The odds of starting up, and driving in the dark go way up with 12volts over six!