Pop your dizzy cap and check to see if you fried your coil. Same exact thing happened to me, even after putting an MSD ignition system with external coil (MSD blaster II). It's definitely the alt. I haven't had the time or money to replace the alt yet, so while I'm driving on the freeway, I drop into neutral and let the car idle every minute or so, for some reason, this won't over-charge the coil and allows me to keep driving without the engine light coming on... If the engine light comes on, I turn the car off, and re-start it. Pain in the ass for sure, but it works.