Had a co-worker back in Memphis that had a 65 Mustang. He bought it new. In 1996 when he retired he said it had 500K + miles on it.
Don't know how true that was. Those old Mustang odometers only go to 99,999.9, so there is no way to verify how many times it had rolled over.
We old farts don't have the best memories in the world, so it's possible it was only 300/400K.
I do know he drove it quite a bit as it was his daily driver for as long as I knew him.
We met in 1980 so I knew him 16 years when he retired. He put between 15/20K on it every year during that time. As far as I know he still has it.
He said he had replaced just about all of the running gear. Engine, transmission, rear end, etc at least once. Some parts more than that.
Body was still mint with the original paint. It was a southern Calif car.