Where is the voltage regulator on a 1972 Chevy truck?

If it is an external regulator, it should be located on the driver's side of the engine compartment in between the headlight and radiator. It has a square cover and a plug with four positions on it.

I don't know if any '72s came with internal regulators. But in any case people do sometimes change the systems to internally regulated alternators. In which case the regulator in inside the alternator.