I'm not sure that it will be a problem for his gauges. Converting to the 69 lamp circuit means replacing one of the pins on the dash cluster connector, but that then will really screw up the circuit card. That RUN-only line handles all of the indicator lamps, so there's some serious problems there. I could easily handle it with the hard-wired dash cluster harness, but that's expensive.
By the way, Ford went back to the more standard configuration in the 1973 Mustang. suggesting that there were some issues found earlier.