You're fortunate to be able to get a Marti Report. They don't have the data for my 65 and 66 Mustangs. On the other hand, I know the two previous owners of my 66 GT Fastback, and have a paper trail for my 65. My 66 hardtop remains a mystery. California Department of Motor Vehicle is forbiden by law to reveal any vehicle history, but a private investigator may get it. A private investigator may be a source you might consider. It will be expensive for you, but can be done.
Jim