It goes around the engine as to avoid the exhaust manifold, turbocharger and everything else that is hot.
Well then, the length & subsequent voltage loss is the main determining factor in a DC system. Seems the shortest length shown in the various charts is 10'. Most show 4, or 6 gauge wire, and that would be assuming that your loads would pull the full 100 Amps, which you probably won't pull.
Do you have big Amp loads in mind ?
Remember, too thin of a wire and it ends up acting like a fuse, heating up & burning through.
Which brings to mind another aspect of wire size, the operating temperature, a component of Ampacity.
Hotter wires reduce the flow of electrons which requires a larger gauge size to keep the flow up without over heating.
Think I'd opt for a single strand of #4.
Also, wondering if you'll find an inline diode on the factory wire just outside the existing alternator?