Foxbodies have VERY weak alternators, barely adequate for a bone stock car. Even if its a "bigger" alt you need to upgrade it to 94-up spec which is called the "3G conversion". Look it up on the interwebs. Fixes a lot of power issues with the foxes.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.