To get that big heavy battery out of the front of the car and over the back wheels you need to buy a kit, this kit will cost around $75, and includes a box with strap to hold lid, kill switch, wire and connectors. Basically you pick your spot, mines behind the driver where the back seat normally is but if you want it in the boot (particularaly for sedans) or whatever thats good, mount the box to the floor using 4 decent bolts with large panel washers to distibute load (imagine force when you hit something and battery wants to take flight)(make sure you put the strap for the lid under the box before bolting down cause its a pain pulling all apart again once youve finished just to feed strap under trust me :-) ). Buy one of thoose cheap battery holder things if youre originals rusted up like mine was (the frame that goes over the top you use to hold battery in place) and a length of threaded rod that fits through it (the battery holder will come with hooks but they are difficult to use here). drill more holes and put the threaded rods on either side of the battery through the floor with one nut under car, one in car inside box to secure to floor and then put battery holder over battery, i put the nut under the car on the end of the thread just hanging over and pooled some solder in the end to ensure the nut doesn't come loose. Now the battery is mounted you need to earth it (make sure its a good earth) and get power from the positive to the starter motor, everything that used to be connected to the positive of the batter you now connect to the positive on the starter motor. To use those connecters they gave you with the kit you need to first strip the end of the wire, then put the connecter upside down in a vice, fill the end with solder and heat with an oxy torch or something to make sure its all liquid then dip the end of the wire in and it should set nice and strong, watch out for the excess solder that spits out the top and dont put the blowtorch down in a hurry pointing it at cardboard boxes  still turned on (once again trust me on this one). The kit does not come with a rubber grommet so you will need to purchase a grommet and drill a hole (13mm or .5 inch) in the firewall and to put the cable through before putting a connecter on the other end, you will also need to purchase some clips which are basically a metal ring with rubber insulation/protection to put around the wire and screw down, its much safer to run the cable through the centre of the car to avoid it getting cut in side impact, but basically when youve figured out where you are going to run it you pop some of these clips around the wire and use self tapping screws to secure it to the floor approx. every 30cm/12 inches. oh and i forgot the kill switch, this needs to put inbetween the battery and starter motor on the positive wire, it just uses a couple more of those connecters, and you have to mount the switch somewhere. Best idea is in drivers reach for convenience but also so you can switch off all power in event of an accident, although with my upgraded alternator the motor continues running off the alternator so you need to be able to turn ignition key off too!. Things like clocks and terratrips ect. are best wired before the kill switch so that they dont need to be calibrated/reset every time you get in the car.

 

And there you go less understeer and more traction to the rear, theoretically anyway, but more likely more room for that engine conversion hey

 

Rally rules will have you putting a hose from the lid of the box to the outside of the car as a vent, but the box is not sealed anyway so i dont see the point, still if you rally youve got to do it.