goddam! how many batteries do you need?! hahaha
never drilled holes in that particular spot, but near it, i was pretty lucky i didnt hit anything cause when i jacked my car up, i barely missed some stuff
best bet would be to jack the rear end up and see if there are any obstructions, itd suck to drill into the gas tank or a line of some sort
if anything, drill SLOWLY and check if you are hitting anything
side note: you ever get your alt load tested? i really want to buy that same one you have that i see on ebay