Should I convert from 6 volts to 12 volts

[tubman]

Positive to positive, negative to negative. It's pretty simple; you have to match the polarity, not the grounds. Needless to say, do not let the vehicles touch.

Another thread just popped up the says to jump directly to the starter lead to skip the rest of the system. That's a good idea; we weren't very sophisticated 65 years ago.