[roysbb]

There is HUGE spike of current that flows from the battery to the starter/block when cranking. Even on the negative side.

If you only use a positive cable from the trunk to the starter and rely on the chassis to carry the return current you add resistance at every junction.
Essentially this return path will perform like a much smaller cable.

Every junction is prone to corrosion and arcing and is not as reliable as a cable connection with its only junctions being at the cable ends (the battery terminal and a large lug to ground near the starter, usually on the block).

Even if you tie the battery directly to the subframe and then from the subframe back to the block - you have doubled the number of junctions in that path. Over time every one of them becomes a potential resistance to current. Resistance generates heat and can melt connections.

This is DC current and will flow through the core (rather than the surface as high frequency signals will). Since this is the case, high strand count is not necessary. High strand count cable is much more prone to corrosion and will fail sooner than a cable using larger sized conductors. The trade-off is flexibility. Seal both ends where the insulation is cut with liquid electrical tape so that corrosion will not start under the insulation.

Possible alternative sources are welding cable, and even high-end jumper cables. I spent almost $80 for a really nice set of jumper cables for one project, they were plenty long and very flexible.
These jumper cables even had the two cables connected by the insulating shield which made for an elegant looking solution.

Hope this helps, Roy