I'd say it's a head gasket, could also be a cracked head. My 75 Chevy had a baffling problem. Fill the cooling system cold, run engine, no leaks, go for a drive, check coolant after cooldown, coolant low. Scrubbed engine, never a drop outside of block, never a puddle under car. Took heads off to do valve job, and discovered hairline crack between valve pocket, and coolant passage.
Thanks for all of the advice guys. Ian's comments were very helpful. I am doing the work myself and don't plan on keeping the car more than a couple of years, so I'm not interested in spending $1500+ to have someone put a used engine in it. That said, I will be sick if the engine craps out after all of this work. I am hopeful that I can fix everything for the price of the head gasket kit.
Thanks again!
-Stephen Smith
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