resistance is all about cross-sectional area of the cable

X2 - flexible is a convenience, nothing to do with how much current it transmits.

Steel (chassis) is an acceptable ground, provided that the X-sectional area is larger than the cable you're not using, and that all connectors have a contact surface at least as large. A 5/16-18 bolt into the firewall isn't enough.

but... why would anyone use smaller than 00 cable for a long run?

Be wary of buying cable at a flea market, or out of the trunk of a car - it may still belong to the utility company.