I looked up the rear interior of a 70 Sports roof in the 70 Mustang Interior Assembly Manual (AM 0032 page 38) and if your description of the area of concern is good, the screw is 55934-S9, 8-18X3/4 oval head 17/64 dia , Phillips, with a bright zinc finish. There are 7 on each side holding the panels, looks to be a commonly used screw.
I recommend that you get a set of Mustang Assembly Manuals to assist you with your car.
Jim