yes i hace given this some great thought. i was planning on mounting two batts in the rear spare tire carrier. i'd spend some time and really mount them good in a well built box/bracket set-up. one of the things about DC current is that the longer the run of wire the heavier the needed gauge of the wire. say your increasing the length of your cables by 12 feet, you might need to thicken them from 4 gauge to 0 gauge (1 ott). of course the safest thing to do is to add up all your electrical needs and then build your cables accordingly..
I also was thinking of mounting two batts under the drivers seat. I'd do this by cutting out the hump and building a box down inside the body. i really want to get my batts out from under the hood, so i can really cut up my fenders.. /forums/images/graemlins/grin.gif