[Shiny]

My timing cover corroded, which had a similar ending. You might consider this possibility before pulling the heads. Do you remember if your timing cover was pitted when you built the engine or is there a chance it corroded through? I hole there will fill up the crankcase with coolant very fast. Seems like a blown head gasket would look like a relatively small leak, and it does seem odd it would be able to fill the crankcase so quickly.