Need help getting a title

This can turn into a real PITA based on my experience. I've got a 65 mustang fastback that I bought 8 years ago from the original owner. I have his bill of sale. I have had possession of the car the entire 8 years. I have tried multiple times over the years to get the car put into my name with no success. DMV is now requiring a notarized signature from the seller... Whom I cannot seem to locate any more. The only snag is that the car was in his name as well as his dad's name. His dad has been dead for decades.