take one HD cable long enough to reach the transmission bell housing if the battery is in the front of car, crimp/solder a battery terminal on it, and crimp/solder a lug on the other end too, now splice a thinner cable from the middle of the original cable to an accesible bolt or stud on the body.
The front of the car/vehicle should also be grounded to prevent electrolysis of the body panel bonnet, it can be done via branching a ground from the engine (fwd jap design) or with a cable at the hinges of the hood (german car design)
Cables should not be oversized as they will induce losses. Also make sure your positive cables are in good service condition and the one from the alternator should not be oversized too as the alternator ranges from full blast amps to trickle charging. Large cables will induce losses here too.