Use the biggest gauge you can afford. welding wire works good but it's certainly not cheap. When i did a relocation on a TJ it was $3 a foot for 2 gauge. you don't need to run a ground lead all the way back to the engine. just run a short cable to a solid spot on the body near the battery and a short cable from the body to the engine.
"I don't have pet peeves, i have major psychotic fucking hatreds" -George Carlin