As long as you have a clean title and registration there should not be any problems if your heirs sell it later. I would just take a digital image of the old engine, prepare a written statement outlining the history and put it with your important papers. But you might not even need that. The folks at the dmv here understand a truck that is over 70 years old is very likely to have a different engine by now. If you had only owned it for a few months that might raise some eyebrows, but since you've owned it for 40+ years...that's kind of a different game. Good luck.
