Well earthing issues are common or what you don't know won't hurt you
I've just placed two earth connectors braided types and the two main places you need earth is near the bonnet lift and one on the negative battery to the solid unpainted chassis leg. The rule is if you have high resistance on the shell you need to make sure earth points are bolted against unpainted metal areas.
Common symptoms are;
Battery drain,
Ecu receiving bad signals due to faulty earth,
Dizzy cap arcing in the wrong place and losing spark integrity,
Alternator power being earthed on to the body via broken electrical wire.
All copper electrical connectors have blue green substances due to oxygenation/water build up on cooper, all you need to do is get Emery paper and sand down the blue and green stuff until you see pure copper.
You can buy the thick cables as you mentioned, but the braided ones are cheaper and do the same job.
They don't need to be huge long cables! Just remember to electrically tape or shrink wrap the braid to stop oxygenation on the metal braid.
Hope that helps a bit into earthing issues.
