Try heating the heads with a propane torch then rap them firmly with a good hammer to jar the frozen stuff loose. Drill a small hole (like 1/8" dia.) above them to allow you to squirt some good lube onto the inside bolt area of each one would also help. .02
If you're using a propane torch try Mapp Gas(R) or Map Gas? (yellow propane type cylinder) as it gets just a little hotter.
