Well, a typical CAT5 cable would have a total of eight 24-gauge copper wires. The size of a 24-gauge wire is approximately 400 circular mills. This gives a total cross-sectional area of 3200cmil for the entire bundle of wires, which is equivalent to a 15-gauge wire. Assuming that you would want to use a #1/0 copper wire for your battery cable, which is 133,000cmils, you would need approximately
42 CAT5 cables to properly relocate your battery. Double that if you want to do the same for your ground.
I would suggest straps like these to contain your hunk of wires going from the front to back of the car. Hang them from the roof inside the car along the door frames for an awesome custom look.