My preference is to use live vacuum hooked to distributor. The extra timing it gives will keep the engine cooler.
The temp sensor in head can give erratic measurements. A better location is in the lower thermostat housing. GM quit putting the sensor in the head (near a exhaust port) for just that reason.
The vacuum can can easily be tested using a live vac line and a timing light. Hook the live line to can and watch the timing increase, unhook it and watch it go down in timing.
