I imagine it would be more efficient to go right to the starter bolt, but that wouldn't be original. So, you need the same 2 gauge strap as you will also need the negative cable to be to carry the massive current of a 6v system.
I think this has a lot to do with most everyone I know's opinion of 6 volts. If you don't change that strap when you change the battery cable, it might not work any better than before. Red's idea is a good one and might save a lot of 6 volt systems from becoming 12 volt systems.
Good to mention here that you need a clean place on your frame for the connection for proper grounding.
