This works well I think, as long as you don't plan on re-using the old bolts.

Use whatever penetrant you like the best (I'm partial to a marine product called CorrosionX), apply liberally, and then grab the bolt with a large (XXL would be better!) pipe wrench (aka monkey wrench) and work it back and forth. You will have all the torque you need and you'll need only a light tap with the hammer to get it to come out once loosened. Worked for me and I bet mine were just as seized as yours.

Good luck if you haven't gotten this taken care of already.

Kai S
'67 Duett