'71 340 Duster has a short

Looking for some help to get Mr Duster up and running again. Mallory dual point distributer and stock coil(I think--round black). Couple years ago I took out the rally dash and had it sent off to a FABO member who did a great job renewing it. I opted to have the amp gauge display like a volt gauge and wired it back in per instructions. Worked great for a year. Then amp(volt) gauge started wavering at idle...went away at higher rpm. One day while out with my 15 year old daughter I did a tight left turn in a cup-d-sac and it died...DARN!! I was just about to let her drive it for the first time. I found a 30A fuse in the main power line in the engine compartment before it goes through the firewall blown. Replaced it and drove it home. Month later took my daughter and 6 year old son out for a weekend drive and during another tight turn it died. I put another fuse in and it immediately blew. I towed it home and pushed it in the garage. I plan to work on it over the next couple of days as we are finally getting rain in San Antonio. I had a new wiring harness put in back in the mid-90's, but I've never done the MAD mod drilling through the firewall plug. I plan to start at the firewall connection and move into the cabin and look for something grounding it...I checked quickly once but found nothing. I had continuity from the firewall side of the fuse holder to ground and no continuity from the engine side of the holder to ground. The wire from the fuse to the firewall connector sure looks small. I'm not running anything that would pull much power...radio is out, heater not connected...just dash, lights, turn signal, etc. I'm up for any suggestions/instructions on electrical troubleshooting to find a short to include how to disconnect the firewall plug without messing it up...I've never taken it apart. Thanks in advance for any help!