The minimum size wire is 1/0 and that’s pushing it. Bigger won’t hurt.
You need to ground the battery to the engine. I know almost no one does it, but in full bodied cars like these, the battery ground cable needs to be the same size as the positive cable and it needs to ground to the engine.
If you ground the battery to the chassis you make the entire car part of the electrical system.
I can’t explain it exactly because I’m not that fluent in electrical stuff but that is the correct way to do it with the battery in the trunk.
If you have some form of ignition that has a box, that box SHOULD be grounded to the battery, regardless of where the battery is mounted.