I read this for setting valve lash/clearance, but had a similar problem. Head leak after running it for 5 years while the radiator cap wouldn't seal.. needed a new radiator but my dad just told my aunt to keep the water up on it, drained it in the winter.. then one day last year the oil pan filled with water.
Fixed the radiator. After reading the thermosyphon part my eyes bugged out and thought, jesus how did we manage to get by with this for 5 years. Tough mudders.
Had the head planed. I'm in IN and strongly recommend B&C engine in Cicero, IN - they are a small shop but one of the fairest and best around.
Added a head shim. It wasn't there either, but it is important to keep the pistons clear of the valves if material is removed from the head. It would likely not have one from the factory, and have seen here or somewhere that you can stack quite a few on top of them. like 3-4. But that would worry me. One worries me.
Only compress a head gasket once. The compression seal holds back something like 18,000psi. Once it has been mushed, replace it - they are like 20$.
New head bolts. Not required but if some guy who knew some guy that calculated the pressure in the combustion chamber is right, there could be strain and wear on those bolts when torqued to 55ft/lbs. They are not as cheap, like $120-140, but I only want to do this once.
Clean the head with solvent, blow out all the ports with an air nozzle, slowly run a threading tool through the head bolt holes in the block, sand it lightly, clean and blow again - check out a youtuber Schodinger's Cat. Longwinded but a good tutorial on being thorough.
Since you took 2 years, I feel better about taking 9 months to be ready to set valve lash and be near turning it over.
Not sure what else - I am trying to fix old problems. Even putting on a alternator it hasn't had for 20yrs.
But, I'd make sure the head gasket is new. That is my guess. If you've been using a new one... I'm going to have to guess the head or the block.
Sorry to hear about the tough go at it, and hoping I'll have better luck.