Is it ok to use the petcock drain hole plus the plugged hole on top GMC head near the 3rd intake port as in/out water ports for carb heat. Both holes are close to the carb location and on the same side of the engine.
You can use the petcock hole from the block for one of the lines - but to mdonohue's point you need to get the other line from the input side of the water pump in order to produce flow:
Keep in mind that the water pump impeller is pushing coolant into the engine - with no flow out of the block until the thermostat opens. If you tap into just the block for both lines you will likely have limited flow (when the thermostat is open - best case) or no flow at all.
The pictures above ended up being a mock-up on my 216. FTF warned me about using brass - so I re-did them in galvanized steel painted the same color as the engine.
This routing produces constant flow regardless of whether the thermostat is open or closed - essentially a bypass loop running through the intake.