How did you look up Optima choices? Their website sucks when it comes to that, you have to check battery sizes instead. Odyssey, if you can find it, is a solid choice. But you will find AGMs under many brands including local stores like Sears, Advance Auto, etc. Most of the good batteries are made by Johnson Controls anyway. So if you stick with good ole lead acid, you might want to look into an Interstate (don't know if they have AGM).
And yes, 850 should have been standard for these cars.