If the block cavity is filling w/coolant, - usually it's either the by-pass fitting /vac cap is leaking (just before the heater core hardline ducks behind the passenger side head) or it's leaking at the double O-rings right behind the water pump.
Ford updated all that with a good fix that I haven't seen fail yet, BUT, only the reman engines received the upgraded heater core hardline. There's a picture of it in one of the Haynes or Chilton manuals....If you own one lol.
BTW, -the HCHL by-pass by the firewall is there for the trucks that have the Cold Climate system package.
Thanks JBrew. From where it's leaking, sounds like the bypass is leaking; at least that's the general location