We welded a 3/8 bolt to the sub frame beneath the trunk mounted battery and one to the front sub frame
passenger side, (we have welded sub frame connectors). 2/0 size cable.

For the positive, ran a 2/0 from battery to an insulated stud mounted on the firewall,
have a homemade screw on cover for that connection to eliminate any accidental contact

I do like the Idea of a lesser gauge ground to act as a fuse and will change one of the ground cables to have that safer set-up.

Thanks for the input ....Joe