When I redid the positive lead in my 35 I ran a thick cable from the alternator to the starter to the battery, when you move the battery to the boot this is the best option as you don't have to run two sets of cable.
You have to have Battery Eearth to body and Body to Block as this is the only earthing point for the starter motor, I've never seen a seperate earth for the gearbox, the reverse switch uses very little current.
Oh and something previously said, the starter motor and alternator work on current, the larger the current the bigger the voltage drop so use the right sized cabling otherwise this is all useless information.