Post pictures of spark plugs. Close up on the strap, straight view of the bottom, and side/angle type view of the strap as well.
A leak down is a way to determine that the motor is indeed sealed up and not sucking in additional air or eating coolant. Use expansion plugs from an auto store on the exhaust, and make two PVC caps to go in the intake boots. One should have a gauge, the other should have a valve stem. Only put 6psi in the motor and it should hold for roughly 6 mins. In all honesty, if a motor has an air leak though, it will lean hang off idle and you'd know there was an issue.
I'd bet you are letting compression blow by in to the coolant system. If you kick it a few times and listen, you may be able to hear it gurgling air pressure in to the coolant system.