Rugo/HrL suggest a welding cable because they might be easier to obtain (you will need correct ends).
Use gauge 00 to gauge 2 battery-connection cables AND all ground cables/straps (all positive/negative cable/strap connectors/cables).
I use 00 gauge on all my 6v trucks - never had/have a starting problem.
Clean all contact/attachment points until they are shiny metal.
Coat with an anti-oxidant grease (NOT a dielectric grease)
Use a copper-impregnated grease, available in small tubes/jars in all auto parts stores).
Your 6v truck/engine will spin/start very nicely, if properly tuned.
Do not consider going to an 8v (tractor) battery.
