This is something we have been doing at the farm for years, with some of the older 6 volt system tractors. Chet hit it on the head, 6 volt system, 8 volt battery, better starts, brighter lights. No mods to the system. Drop it in and go. We buy them locally at fleet farm, and tractor supply has them as well
Here http://www.tractorsupply.com/en/store/exide-heavy-duty-farm-battery-8-1
Here http://www.tractorsupply.com/en/store/exide-heavy-duty-farm-battery-8-1