Here is what I used :
The main cable from the rear is 00. I used a wire clamp to secure it. That line is very important to insure proper routeing and securing, as it is connected directly to a batt. In some parts of the routing, I used rubber heat shield. It's important to note in this kind of install, that routine inspection of main lines is needed.
Once the main 00 Power line is in engine bay, I connect to a power distribution block.
The 4 large power lines :
1. To factory fuse block
2. Wench
3. custom fuse and relay distribution center (off-road lights, Primary electric fan power
(I use a custom triple redundant, dual automatic thermal control circuit to power the cooling fans. 12V power is supplied va 2 sources of power on self resetting circuit breakers.), relays, and other after market equipment that needs power.
4. large cable to starter motor
I also extended the ground. It connects to rear, near Batt, connects to body (with a stud that coes through for AMP hook up), and then connects to front frame. At that same point, a very large flex ground to the engine (4 AWG)
When most people relocate a batt, they don't apply as much focus to the ground. When in fact, the ground is just as important as the hot, if not more so.