Answer: The reason that you are having difficulty finding an aftermarket wheel that will fit your car is because 1965 Mustangs that were equipped with 6-cylinder engines were also equipped with “4-bolt wheels.” If the car was equipped with an 8-cylinder engine, then it was equipped with “5-bolt wheels,” which are much more popular.
I don’t know of any aftermarket wheels that are produced in a 4-bolt pattern. There are adapters available that can convert your car from a 4-bolt to a 5-bolt pattern but I don’t recommend using them.
You could convert the car to accept 5-lug wheels which would offer you a wide choice of wheel options. To do this you would have to convert the rear axles and the front drums to a 5-bolt pattern. The good part is that parts are readily available. The bad part is that it is not an inexpensive undertaking.
Many people contemplating the same issue find that this is a good time to convert to disc brakes, as the cost of the parts are only slightly more, and the disc brake kits are available as 5-lug assemblies.