'71 340 Duster has a short

If the fire wall side of the fuse holder is going straight to ground, personally I’d set it up where I could watch the meter (assuming that’s what your using) and start pulling fuses from fuse box to see if ground goes away. I’d try not to disturb anything else until I determined what electrical branch off fuse box made the ground go away. Not saying it couldn’t be prior to fuse box. But that’s where I’d start.