My son, give me thine heart, and let thine eyes observe my ways. - Proverbs 23:26
For as he thinketh in his heart, so is he: Eat and drink, saith he to thee; but his heart is not with thee. Proverbs 23:7
Thy heart is not right in the sight of God. - Acts 8:21
Isaiah 43:1, (KJV), But now thus saith the LORD that created thee, O Jacob, and he that formed thee, O Israel, Fear not: for I have redeemed thee, I have called thee by thy name; thou art mine.
I agree that there is no head or heart distinction. However, if you haven't preached to the heart of a man, you haven't preached to his mind, as the mind contains the heart. And if a man's heart has not been turned (Acts 8:21), it means his mind has not either as it is the mind of the man that follows his heart (Prov 23:7). So you can't preach to a man's mind unless you preach to a man's heart.
Furthermore, you can't believe the truth in your mind unless you believe in your heart. The scriptures are quite clear that it is the heart of the man that God is after! (Prov. 23:6) And anything less than belief in the heart like Simon the Sorcerer who just wanted knowledge and special abilities (Acts 8:21) will leave the man an empty shell who may find enjoyment in playing theological games for a season having never truly understood. And without belief in the heart, he has never enjoyed conversion, nor has he enjoyed the sweet whispers of Christ to his innermost, "thou art mine!" (Isa 43:1)