oh crap... how many miles has this been going on?!? Let's hope the bearings haven't been destroyed by that yet.
It must be something cracked, or a very strategic HG failure leaking into the cam housing drainage passageways. What came out when you drained the oil???
Luckily, a good used NA head is not terribly expensive. Finding a
good used block can be a PITA depending on your standards. If it was any of the other more common oil/coolant mixing type failures, it almost certainly would have been leaking oil into the coolant before this. The oil works it's way into the expansion tank, so that would have been obvious.