I believe a factory option for 65 was rear set belts. You may already have the threaded anchor points to install the seat belt bolts. Pull out the bottom seat and look on the drive shaft tunnel and corners of the lower panel. If you see any rubber drain plug looking items pull those off and see if there is any threaded anchor plates.
Iin the early 1965 mustangs, belts were attached with eye bolts, in mid year they switched to a standard hex head. If you have them all you need to do is get some new quality belts like from
Mustang Parts House . You don't want belts that are old and dry rotted from sitting out in the junkyard. In a real emergency thos could easily snap.