I'm no experts when it comes to seatbelts but I did notice that the anchor plates are not what I'm used to seeing on early Mustangs. Made early in '66, they appear to be for a full size Ford, but again, no expert.
I've attached a photo of what the rear seatbelt anchor looks like for a '66 Mustang; note the sharp bend in the anchor. I also included a photo of the anchor typically used for the front seatbelts; they're flat, no bends.
Hope this is of help and since no one else has replied, you may want to move or re-post your question under the 1964-1/2 - 1966 category, you may get more and better responses.
Good luck
Ray