I just wanted to come back and say thanks for all the help and to update you on my final results, almost a year later. Thought you guys might like to know how the fix went down.

As earlier confirmed, with a chemical tester, there was a small HG leak, or a cracked block. I think the former, given the mileage. I scoped the cylinders and it turned out that cylinder 5 had a small amount of coolant in it. The plug also looked a little cleaner than the rest.

Anyway, I used this K&W block seal, making sure I isolated the heater core first. The first time, I followed the instructions and idled with it for 20 minutes, but it didn't do squat. The leak wasn't big enough for the product to make it's way into the fault area so quickly. I tried another bottle, this time I ran it in the motor for 500 miles and that did the trick. Almost a year later and the coolant stays filled to the neck.



Thanks again!