Some thoughts on dual battery installation.
Firstly, the location needs to be vented - above the battery. I don't see how this can be done in the wheel well.
The battery needs to be firmly strapped down. Shouldn't be too difficult.
I really wouldn't want to lose the spare wheel for a second battery. That tyre repair kit might or might not work for a nail that goes straight through and is dealt with quickly, but all too often a nail will not leave such a neat hole by the time the tyre has slowly gone flat.
I think I would probably go for one of those "jump start" type portable batteries, find somewhere in the boot to strap it down. More versatile, and certainly capable of starting a V8 with a totally dead battery if you get a decent one.