I had the exact same problem at the end of the driving season last year with my '79 SC.
Started with the tach bouncing, intermittent alt light which became brighter with driving, e brake light difficult to go off then finally would not go out at all. Then after start up one evening while waiting for the engine to warm up, a line of smoke came up from the defrost and the smell of rubber burning. I subsequently shut down the engine and it has sat until this morning when I uncovered to assess where to start to fix.
According to 101 projects for 911, it recos repalcement of the alt. Guess there is no other option.
Have you sourced one out yet?