Do not hook the negative to the battery. You want your ground as short as possiable. Somewhere around 18 inches is fine. Instead hook it to the chasis, and make sure to remove any dirt, or paint before you do.
Being a little longer isn't going to hurt to much. Just remember your ground is where you pick up the most noise. The longer your ground the more EMI it will pick up. Something alot of people don't realise is that electrons move from ground to positive. Thats why all the big dogs in here sugest to upgrade your battery - to ground, along with the alternator to battery positive. You can even go as far as upgrading the wire that goes from the engine to ground.