Sorry to split hairs, but there are no "body mounts" on a early B-body.

The factory engine mount "insulators" provide an electrical insulation too. If you have steel mounts or motor bracket you're good.

The K-member is directly bolted to the "Uni-body" framerails that has innumerable spotwelds tying them into the body.

The thin little "groundstrap" that goes from the engine to the firewall is really for the ignition electronics like distributor spark. Even though it's high voltage, it is actually relatively low amperage and that's where big thick wires are needed elsewhere.

Hence the stock battery ground went straight to the engine block/head and the positive cable went straight to the starter which ground to the block too.

As you add length to the cables, you add resistance, so cable gauge needs to get thicker/bigger (number gets lower).

Adding a remote starter relay further complicates matters. If there's internal resistance from low voltage cranking, burned/pitted contacts you're hosed.

Simple things like the size of the terminal/lugs and quality of the crimping/soldering done to transfer current can strand you. And as most know rust/corrosion and loose connections are bad too.

Keep it simple and go overkill.