This statement has already proven without a doubt that you have compression leaking into your cooling system…that compression also has fuel in it because it needs to have this before compression takes place.
Remove head, take it to a machine shop to be “decked” or at least checked for “square”…then proceed to do the rest of the job. I have done more cylinder head jobs than I can possibly recall, and not a single one has had any issues, because of the formula I outlined for you. The people who do not do these things are the ones who bring me their cars and ask me to solve the problem.
Its just a little hard won, battle proven advice…ignore it at your peril. LOL… No…but I’m serious…