I put my battery in the trunk. I initially ran 0 gauge wire from the local farm store. I had lots of problems with voltage drop during cranking on start up. I found my problem to be not enough strands in the cable. I switch out to welders cable of 00 gauge (larger) with a LOT more strands of wire. Voltage drop was greatly reduced. Strand count is as important as diameter.
I ran my cable through the trunk along the quarter, came through the rear interior panel at the floor pan, and out of the interior in the floor plug just behind the passenger seat, along the frame to the engine compartment, and along the firewall, to the starter. I don't know if it right, but it isn't shorted, or burning on anything. If you plan on racing, some organizations have requirements, so follow their rules.
I did the alternator/ammeter bypass at the same time.