The instrument voltage regulator causing a dash fire is very unlikely. The copper traces on the circuit boards serve as fusible links, burn to open.
Smoke from under the dash usually comes from the big wires on the amp gauge. At some model (around 74) they went to a remote shunt design so the same small gauge wires for everything else were used to power the amp gauge. They even went through the same harness connector rather than bolt directly on the gauge.
My memory fails. That change could have been 75 and 76 only. You will remove the entire cluster before you find the problem. Good luck
