Yeah, all finished a few weeks ago.
I discovered a few other cabling issues and rectified them along the way, as well as replacing the starter motor.
In short, for the extra battery:
I ran 000 gauge cable from the isolator, mounted in front of the brake booster - down the chassis rail to the rear RHS of the vehicle, then up into the space behind the wheel arch in the boot. Battery is in a sealed marine enclosure and secured to the side and floor. There is an isolation switch directly next to it as well as a number of circuit breakers for specific feeds.
I also installed some auxillary LED reversing lights under the rear bumper for night work at the boat ramp.
I have a couple of switched anderson plugs, (50A, 120A) as well as an isolated bus feed for other future accessories. There is a lot of work involved in all of this, but worth it.
I'll take some pics when I get time, as I will have to dismantle stuff to get appropriate pictures.