Gday mate,

Just been through a battery relocation myself.

Firstly i would run a 0gauge positive multistrand to your starter motor, then a 0gauge negative to a ground point in your boot (close to your battery). I would also run a secondary negative to your engine bay - 2/4 gauge (this will help minimise voltage drop over your chassis). Make sure you use high end double insulated multi-strand cable (not 7core or solid).
I would also highly recommend installing a inline circuit breaker between your battery and starter motor (dont waste your time with fuses - they are a nuisance - check out Jaycar electronics). If you are ever in a serious accident and your cable breaks and comes in direct contact with your chassis it can result in massive fault currents ! Also a circuit breaker can be another point of isolation to help keep your car secure from thieves (they cant hot wire the car if no power to starter motor and they dont know where the switch is ?)

Make sure you use a drycell like odyessey or similar securly mounted in a battery bin (check out battery world - Dont waste your time with supercheap)