The advice that I remember on the VCCA site for boosting a 6v system from a 12v was:
Hook the positive starter feed up to the 12 volt system. Take the ground from the 12v system and clamp it on a remote part of the frame or bumper. Then start the truck and immediately remove the 12v connection. This is a 2 person job working in close coordination.
I think the theory here is that there is a voltage drop across the poor ground connection. I would suggest not running the 12 v engine while this is going on.
