Go to any welding shop. They sell SJ heavy duty wire and all solid copper lugs. My welding supply will sell any cable they carry by the foot and they will swedge on the heavy duty lugs for free if you buy all the stuff needed right there. The local welding supply sells #4, #2, 00, and 0000, and the solid copper heavy duty solid copper lugs to go with them. This is serious oil resistant cables with very fine wires bundled within the SJ jacket making them perfect for use where easily flexible, oil resistant, abrasion resistant, cable is needed. If it were me, and I have been there often, use the 0000, and go from the negative battery connection to the frame and another from the frame to the engine. Add in another 00, from the frame to the body and you will never look back. Remember that grounds complete all the circuits back to the battery in DC and they are often overlooked when you have any weird electrical ghost. Over sizing grounds is a cheap and often needed electrical gremlin fix. Be sure to abbraid the surface to get down to shiny metal, then coat with dielectric grease and tighten down with a regular flat washer on top of the lug and a star washer under the cable lugs between the cable and frame, thus assuring a good electrical and mechanical connection.
-Paul R. Haller-