I am so glad you have found the problem before disaster and I am so sorry you had to pull the engine because it is a lot of work.
But! At least the count down is on to going back to a happy running engine!
A lot of people don't understand this but it has to be done. keep your coolant at a PH of 7 or close to it, over time it will change. 1 is acid, 14 is alkaline, both will eat out your freeze plugs. Don't just check your coolant with a antifreeze tester and call it good, it might not be.