if you use the new optima battery or some of the other sealed batteries, you can mount it upside down if you want....not that you would, but they can be installed however you want ....
Be sure to use the correct thickness of cable for your positive. For i made this mistake and my alternator wouldnt charge my battery properly. Also if you go under the carrage make sure to stay away from hot exhaust pipes, the drive shaft, or any area where the insulation could rub away and ground the positive out. Good Luck.