I had mine mounted between the rear axle and body, in the worst possible environment where it got wet, and it still worked fine. Just run something like 2 ga battery cable to the starter for the positive, and also run another 2 ga cable from the negative post on the battery directly to a bolt that holds the starter to the block. That way you have a perfect solid ground and get full juice to the starter. You will still have to run some grounds from the engine or transmission to the frame and also into your wiring harness, but at least you will have full power to crank the motor if you do it that way.
Don
You can see the battery mounted on the right side of my car, in front of the rear end.