But I have found the leak - small crack in the head, coolant enters above no 5 exhaust valve.
When my engine was professionally rebuilt by Metric Motors, part of the process and procedure for all their rebuilds is a detailed crack inspection including the head. They don't need to know or care about what condition the head is in when it arrives, but only when it leaves. Sure enough, on mine they found a crack. Luckily it was small and repairable. They welded it all up and did whatever they had to do after that in their process. It's been about 13 years now and all seems to be well.
Others I know have sent their engine in for rebuild, too--only to find out that yes there is a crack, and no--the head is either too far planed already, warped, cracked or whatever. Sometimes these things are not obvious except to the pros with the tools of the engine rebuilding trade.
Hope your crack can be easily repaired!