uh oh..... set back. water in oil

Stock rebuild? Antifreeze in it? Is so at this point don’t go crazy cleaning it, you don’t know where its coming from and the antifreeze will stop corrosion. Did you see it run before being rebuilt? Its tough now its apart, you can’t run compression test or pressure test cooling system to try and locate leak. Really if you are sure the head gasket was good and the intake seal was good you are left with a cracked part or timing cover leak.