Hi,
If anyone may benefit from this, we have been using Group 24F batteries for at least 8 years on our 850s.
The Group 24F:
--is of higher capacity and Cold Cranking Amperage than the direct-fit battery.
--is cheaper to purchase, and sometimes much cheaper than the original size.
--usually going to be much newer (by build date), as that size rotates in store stock much more frequently.

The Group 24F battery footprint is easily within the base plate (by about half an inch width-wise), and the 850 cables fit. (The outboard clamp--near the fender--fits with no height modification; for the inboard one, just remove the single, 10mm bolt that holds cable retainer to the base plate and instantly you have the freedom to put in the taller battery. There is still plenty of slack in the inboard cable with the clamp removed, even about another 2 inches.)

Since the battery is a universal 12-volt, the electrical system is not hurt at all. (The car draws only what power it needs).

We use the Consumer Reports top-rated Sears Die Hard top-of-the line model, different for the regions of North and South, which even goes on sale sometimes.

Hope this may help...