Run away Ford C6

Has anyone else lost their Ford when it jumped into gear?

I had a mean looking 67 Galaxie 500XL flat black, shaved door handles, headers, stock car style side exhausts. I started the engine and backed out into the driveway behind my house. I got out to move my daily driver while the Gal warmed up. As I walked away I heard the rpm change. I turned to see it going over the hill. I kept waiting for it to get hung up on small trees or brush but it just kept going, going, TREE. Fortunately Hagerty took great care of me and now I have a 71 LTD convertible with the same sloppy transmission.
What is the cure other than never
leaving the driver’s seat?