I've been there too.
I am afraid to tell you that you have quite a problem on you hands. This is a difficult job for a master Cadillac machanic, yet alone any others. What most likey is going on is that you have a cracked head bolt. This can happen when the head gasket seals go bad. What will need to be done is first finding which head bolt is broken and then the bolt will have to be drilled and tapped out from which ever part of the block it is located at. This requires the removal of the engine from the engine bay. There is a youtube video of a guy doing this while the engine is still in the car but even my master Cadillac machanic is not sure how he is able to do this properly. The job cost me about $4,300 plus the fix of a leak in the oil pan. I have heard of it done for about a $1,000 less but for me I wanted to know that it was done by the best and thus I went to the Cadillac dealer. I hope the block isn't cracked which is possible since it kept overheating on you. If the body and interior are solid on your car then go for it. These Eldorado's are becoming more rare and we need to keep them on the road in order to keep the Camry's from taking over worse than they already have. These are really fun cars as you know and deserve the work that they require as the miles continue to climb on them. Besides what kind of car can you buy for around $4,000 and the engine should be reliable for quite a long time after this job.
