Hey Blink,
I fitted lap sash belts in my 68 Mustang.
Not sure if 65 66 Mustangs are the same.
On the C pillar right above the quarter window there is factory holes, a slot about 1” x 3/4”, about 9/16” hole and a 1/4” hole, these are used to fit the top anchor plate for a lap sash belt.
You slip the anchor plate through the slot, anchor the plate with a bolt through the small hole and the seat belt top anchor screws into the large hole.
Just lightly push against your roof lining to locate these or pull the lining down a little to locate these.
Email phillipdrury@icloud.com if you’d like a photo.
Once again I’m not sure 65 and 66 have this arrangement!
If you attach the seat belt at the bottom of the window it is too low and you will find it uncomfortable.
I also used a dropper with my seat belt.
Cheers Phil.