actually this exact thing happened to a friends f body. im guessing this has the same 3800 series engine?

anyways, have the rad pressure tested. chances are it just overflowed from the reservoir. he took it to some garage in hamilton mill and they said head gasket, new head, blah blah blah. they went to another garage in the area and the guy just replaced the thermostat, and everything was fine. its definitely worth a shot. 30-40 bucks beats 1500+ any day. if nothing else you can just clean the thermostat up if it doesnt work and return it, and you arent even out of money at that point.