The info you got from the "local Mustang guys" were for a hardtop, not a Fastback, so disreguard them completely (and get some new Mustang guys).
In your first picture, there is a screw that goes in a hole at the end of the trim piece that goes between the vent panel and the panel with the light into the body. If you look closely at your first picture, there is a hole in the upper panel for the screw to clear, and I think the hole in the body in the second for the screw (although it looks enlarged a bit). My two pictures show the trim installed on my "K" car where it wraps around to the edge of the panels. Your trim piece appears to be too short as well.
I would recomend you get a set of 66 Mustang Assembly Manuals available from most online Mustang parts sellers. You will need 66 Interior, 66 Body, 66 Electrical and 64 1/2 Chassis (there is no 65 or 66 Chassis). There are two optional manuals, 64 Weld and Sealant (again, no 65 or 66 but 65 Fastback is there), and 66 Engine Accessories. The disclaimer, those manuals are at least 90-95 percent accurate which is better than "0".
Jim