When you extend it, just use cable the same thickness. I used winch cable from ebay, iy was more than up to the job. multi stranded copper.
In the engine bay, I bought a large connector block and jointed the 3 cables where the +ve battery terminal used to be. You don't need to do anything with the -ve cable. You could remove it and run a separate cable from the gearbox earth to chassis earth.
Then in the rear you only need a short cable on the battery to ground it.
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The rain kept a trollin'
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