Ammeter pegged...

I had a similar issue on my 72 Satellite. The ammeter would peg and the lights would get very bright. Sometimes it would be OK. For me, it turned out to be the voltage regulator however I would follow the advice given here to troubleshoot. I would also agree and be very careful with that situation and not drive the car until fixed.
In my case the battery started to bulge and the bulbs got very hot. I got lucky.
I also keep spare VR, ballast resistor and a spare alternator in the trunk.
Good luck and keep us posted.