I have heard of people boiling out old rads but have no experience of it and having looked at the clogged tubes in mine, I seriously doubt any cleaning would have been very effective. When the tube is solid with crud there is no way for anything to work it's way through to break it up.
If you get a cheap IR temperature gun and run the rad up to full temperature, you can scan it with the gun and see where the hot/cold spots are to give you some idea of how bad it was.
My rebuilt 5 core was $500 at a local rad shop by the way.
