Lights don’t dim with more electrical devices on?

Dana67Dart said

IMHO it was coincidental....Unless the instrument cluster in a 74 is VERY different than a 67 (yes I know it is physically different, I mean electronically) the instrument cluster regulator ONLY feeds the fuel gauge and the temp gauge. The instrument panel lights are 12 V and get their power from the dimmer in the headlight switch. so changing to a solid state instrument voltage regulator would have no effect on the dash lights AND the headlights as the OP stated.

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The power from that regulator only feeds those two items but you have to understand how the regulator works. The regulator is dumping power 7/12'ths of the time, to provide 5V for the instruments. The electromechanical regulator is literally grounding itself, repeatedly, all the time, and the grounding is causing the load variations in the charging circuit.

It is the same principle as the vehicle regulator, and the same net result when that is replaced with solid state, which does not fluctuate at all.

AND your gauges will be more consistent.