How does one do teshuva on gnevas da’as that happened over 40 years ago? It was kind of intellectual theft without any monetary damage.
It is unclear from the question exactly what took place all those years ago. If the “victim” was caused pain or distress, you should ask forgiveness (if he is no longer alive one may do this at his gravesite) but if not, there seems to be no need to approach him directly after such a long time.
If by intellectual theft you mean that you stole an idea of his or a trade secret, it may be necessary to pay monetary compensation. A Dayan should be consulted with the details of what took place.