I won't hook a 12 volt battery up to boost a 6. Heard too many stories of exploding batteries sending acid all over the place (and people). I've used 12 volts to start my Merc in the initial stages of it's resurrection when it had carburetor issues and a few stuck valves. But I by-passed the 6 volt battery. As has been said, hook up the booster cables direct to the starter side of the solenoid and ground to a head bolt. Polarity does not matter as the 12 volt will just spin the starter. I did the final connection at the 12 volt battery. All by myself. Its on youtube.
https://youtu.be/wO25Y2zgShE