More probable causes:
Blown head gasket(s)
Warped head ( more common on aluminum heads)
Cracked head ( usually indicated by oil/water mixing in crankcase)
Cracked or split cylinder wall ( absolute worst case scenario, block replacement or sleeving required)
I would run through the list of things deuce gave you before moving on to these. If all else checks out, pull the heads and you'll usually have your answer.