It not 100% necessary to paint over it, its just to insure it doesnt rust or corrode over time.
Ive never painted over mine, but I probably havent kept the same system or wiring long enough to see rust/corrosion ever be a problem.
I would run a single 1/0 from the front batt + to your amp/battery in the back if you have one, and then ground everything else to the frame.. use a bolt that is solid, if you an wiggle the ground its no good. Sand it down first.
I usually do the big 3 like this.
Alternator + to front battery +
Alternator mounting bolt to front battery -
Chassis to front battery -
The alternator mounting bolt will suffice as an engine ground.