You might also check the freeze plugs, there is one in back of head also. Check the radiator, tiny leaks can go undected for years, as well as the coolant bottle, it can lead and you never see it. If you want though I am not sure if it will work, is to pull the plug rail off, pull plugs out and spin engine over and shine UV light into each cylinder and see if you see anything green in there. You might want to put each piston at TDC for this but if there is a leak in the head or gasket it will get inside and show up. You could also first start up engine let it idle till water leaks out the exhaust. Then check that, not sure if it will work but possibly dye will not combust so it might show up in the water from the exhaust. If you do this you'll have to let the engine cool overnite to do the first suggestion. These may not work, but have had coolant leaks in the past that were small, constant, and seemingly invisible in a engine that ran fine for all the years I had it. Hate to see you put on a head, then find you still have that leak.
Oh and check heater area. There is a drain, check around it. Some of these leaks are super sneaky. My old Ford Ranger will every now and then leak coolant out of the heater hose connections at the body. It will drip a bit on the ground for a day or so, then go away for who knows how long before doing the same thing again over and over. Heater core is good, hoses are new, has done if for ages.