I'd actually be wary of fitting a battery inside the cabin, particularly if you're a smoker. While charging, even sealed batteries can release hydrogen (it's rare for a sealed battery to do it, but it has happened before and will happen again).
If you light a smoke, it'll be the Hindenberg all over again.
Lead acid batteries belong outside the cabin, in my opinion. Some place where the hydrogen can be vented off safely. Yes, there might be room for it there - maybe you could put some extra tools in there, and fit a battery tray on top of the wheel arch in the tub, if you don't want to use the space in your engine bay.