I added 6 gauge wire from the battery to the chassis, from the rear of the block to the firewall, and lastly from the drives side of the engine to the area near the fuse block. You could obviously go larger than 6ga but had plenty on hand at the time and it still has a factory appearance.
Made sure to use sand paper for a clean surface 100 or 220 grit will work fine. Left the all the factory grounds in place.
Picked up 0.38 volts took about 30 minutes and was worth it in my opinion.
Try to find high quality solid copper wire 6AWG or larger with a thick insulator, get copper ring terminals and make sure to get a solid crimp. Heat shrink from the wires to the ring terminals will give a nice clean finished product.