Thanks for the suggestions guys. I may have to go with something like that. I tried a couple batteries in the Metro this morning and it is looking like I have a height clearance problem.
Here is the stock battery.
Here is the stock battery next to a group 31 battery, what I'd ideally like to put into the Metro. Slight size difference. The thing you can't see in the picture is that the group 31 is over an inch taller.
Here is the group 31 battery in the Metro. Looks almost comical IMO.
However, this is what happens when you try to close the hood. So, the group 31 battery definitely will not fit under the hood. Bummer.
Now, here is a group 24 battery I had kicking around. I managed to get it under the hood and the hood even closed. However, its really only on the tray a little of half way, so I'd have to modify the battery tray to hold it properly.
So, I'm looking at a few different options at this point:
First off is seeing what kind of capacity I can get in a group 24 lead acid battery. If its 80-90 I might just settle for it as it'll be the easiest thing. Modify the battery mount and be done.
The next option I have is going with a smaller battery up front, and sticking a group 24, 27 or 31 in the back somewhere and using a DC-DC converter or something as Mechman stated. I like this option as it runs a more normal voltage while driving. But, honestly I didn't have any issues running without that on my Paseo when I had an alternator delete. I'd also need to find a DC-DC converter capable of pumping out around 40A which isn't a tiny amount of power.
The third option would be go size a ~100Ah lithium pack as redpoint5 stated. This would be the most expensive option, but it should theoretically last quite a while. I'd need lithium cells and a lithium charger at the least, possibly a BMS.