Cracked heads? Could be a hairline crack that allows enough coolant to leak into the the chamber until it gets warm enough to close up the crack then it's OK. Hence the steam (excessive pressure) Have you looked down into the cylinder with a light to see if there is excessive carbon or is it super clean like it's been washed? If it's super clean, it's getting water washed. Is this a 360? If it is, it probably has '308' heads? I believe they are known to crack between the valves. Could be a dropped valve seat?
Other thing I can think of is something happening during the warmup cycle, like detonation or an overly lean condition until the coolant temp gets to a certain point then the a/f mixture stabilizes until the next cold start. I know nothing about early computer controls but perhaps you have an electrical/computer issue?
Have you checked your coolant temp sensor for proper operation? Done a leak down test? Checked for a vacuum leak that closes up after it gets warm? All kinds of stuff to do before you start replacing parts.