Howdy All!

I would really appreciate some help from all you knowledgeable Caddy people.

My name is Jim. I recently purchased a 1981 Cadillac Deville, 9 passenger, 6.0 liter engine. It's got the 4-6-8 thing.

I am having a lot of difficulty getting this thing to pass smog. It's got 52,000 original miles on it. I replaced the EGR valve and the front oxygen sensor. It went from 2500 to 1500, but only 1000 is allowed.

I did a bit of research and it's possible it needs the MAP sensor cleaned. First, is that likely and second, is it possible, and third, where is it and how do I get to it?

Could it be anything else? I went to a shop here and they quoted me $109 to start and it goes up until we find what's wrong. In other words, there is no upper limit, so I am a bit hesitant to go there.
Thanks for all your help and suggestions, in advance!