Optima's website is junk, so I would try calling them directly and see what they say. Supposedly, they are building an application guide on their website, but it isn't finished yet.
I think the OEM battery is a heavy-duty group size 31 battery, and so I would shoot for this one:
The yellow top is for deep drain (very high powered sound systems and other deep cycle implimentations). The red top is the HP versions for quick starting in the cold. Autozone has free shipping (as I recall) and great prices.