twin battery 88 HJ75

Royce
Normally in a HJ75 the batteries are mounted end to end, there will be plenty of brackets available commercially or you can make one up yourself using angle.
As far as isolators are concerned you can do it manually with a marine type switch, or you can use a solenoid (continuous duty) that marries the 2 batteries once the ignition is on or you can put a battery management system in the middle to carry out this process for you.
In my last vehicle (a HJ75) I used a low voltage cut out (available from Mr Smiths electrical stores) to only power up the solenoid after the main battery was at a set voltage, this unit had the advantage of a push in button that would power the solenoid and join the 2 batteries if the main was low or flat rather than having to use jumper leads to start the engine.
Regards

royce, i have an 88 75 series. my suggestion after many problems, like regassing the air con 5 times, is to have a really good look at how the brackets and battery relate to the compreesor for the air con, I ended up ditching the original tray and fitting a double, this helped but it is still very tight in regards to the air con hose that leaves the compressor and has to turn 180 degrees, with all the vibration going on even the slightest rub will prove to be very costly, at least i always end up with a fully charged air con. Just yesterday we regassed again, this time trying a swivel fitting to take hose away from the battery but am now dangerously close to a rib in the bonnet, we'll see what develops.
My main motivation for fitting a double was the condition of the original bracket, it looked sound from above until the battery fell through it one day on the way to landcruiser park and shorted out the ignition loom, (how many people get towed INTO Landcruiser Park) with the fresh smell of burnt insulation. It may pay to really check the original, and replace with a double.
Hope this helps, Dave