I hope that you didn't add pure anti-freeze instead of a 50/50 mix (preferably pre-mixed by you in two anti-freeze bottles or 1 gal containers)? If so, you need to drain it off and correct that because pure coolant will freeze.

After initially adding coolant to a drained 850, usually there's a big air pocket trapped inside and after overfilling the bottle you need to test drive the car (high RPMs) a few blocks after the T-stat opens, then re-check the level if more coolant is then required, slowly and "CAREFULLY" open the bottle as steam and hot coolant will boil out if opened too fast (best done with engine running). Then let it cool off a while and recheck the level.