I know this has been discussed at nauseam, but I want to make sure and get more info. It looks like the upper crossover tube between the left and right heads has a plug in it with a square fitting. I just want to confirm that I can unscrew that. That seems like a good place to refill the system? I don't know how else you can get coolant back into the heads after draining, at least until it gets to full operating temp and the thermostat opens???