0 gauge wire and as for ground depends on the car. If you have a solid frame vehicle and you can connect the battery to in the trunk and then again in the engine bay then no, if not then yes. If you run both ground and positive run 0 gauge for both.
Thx, Car is grounded; I had this in mind for a long time; at first thought of having two batteries one in the trunk and the engine bay. I'll get a battery housing and secure it.