Ammeter pegged to plus on my 74 318 duster?

There is a "main" power wire running to the fuse box that may have been burned during your dash fire that is intermittently shorting to the dash , firewall , or somewhere else. I had this exact problem with a '69 Satellite. mine shower it's self by shorting out several voltage regulators ( the older point type regulators). Took me forever to find it due to it running along the lower lip of the dash to the glove box fuse panel. I would be looking at that wire first. David