Jesus refers to himself countless times in scripture as the "son of man". Well, not "countless", but 84 times. He is referred to as the "son of man" in all 4 gospels as well as Acts, Hebrews and Revelation.
In fact Acts explicitly identifies Christ as the "son of man".
Act 7:55 But being full of the Holy Spirit, he gazed intently into heaven and saw the glory of God, and Jesus standing at the right hand of God;
Act 7:56 and he said, "Behold, I see the heavens opened up and the Son of Man standing at the right hand of God."
So on scriptural grounds Jesus Christ is indeed the son of man and Matthew 12:40 is indeed referring to Jesus Christ.
As for the "heart of the earth", a more credible explanation is that Jesus used language familiar to the scribes, pharisees and increasing crowd.
Jon 1:17 Now the LORD had prepared a great fish to swallow up Jonah. And Jonah was in the belly of the fish three days and three nights.
"Belly" is:
mê‛eh
may-aw'
From an unused root probably meaning to be soft; used only in plural the intestines, or (collectively) the abdomen, figuratively sympathy; by implication a vest; by extension the stomach, the uterus (or of men, the seat of generation), the heart (figuratively): - belly, bowels, X heart, womb.
The same word is translated "heart" here:
Psa 40:8 I delight to do thy will, O my God: yea, thy law is within my heart.
So Christ was making a clear comparison between being in the "heart" (or middle, center, bowels, etc.) of the fish, with being in the "heart of the earth" and was using phrasing that would convey the meaning more accurately and make more of an impact to his listeners.
Sigh. I never made the claim that Jesus did NOT ever refer to himself by the phrase "son of man". I already had to point that out in post 231. I get the impression that the number one goal here is for people to shout past each other in order to push their pet doctrines.
I was trying to point out how traditions and doctrines risk going awry, when people assume and cling to interpretations that are not demanded by the text. Kind of a Bible study version of verifying one's scientific methods: searching for and analyzing places in the text where experimental error is possible. It's tedious and it sure irritates people, but oh well. That's life.