That main power wire is probably what caused the dash fire, and it sounds like it hasn't been dealt with. Right now, it's probably dumping max alternator output straight to ground, hence the maxed gauge.
Disconnect the battery so the car doesn't burn down while you're asleep, and FIX THIS. The main power feed passes through the bulkhead connector (firewall) and are NOTORIOUS for getting corrosion in that connector. Corrosion=resistance=heat
Once the wiring is fixed (No electrical tape...actually fixed), run a heavy-gauge wire with appropriately sized fusible link from alternator output stud to battery. This makes your ammeter worthless, but you can't read an ammeter through the smoke and flames anyway.
