What I've found, mostly from San Jose cars, is that there is mix of parts used. Based on the size hardware required, all 7/16 inch, the selection of the following seems to be reasonable; there are few common assembly line bolts or nuts of this size; Ford did not want special hardware; Ford may have bought cheap bulk quantities. The only documentation for tie down plates in in the 71-73 Body manuals referencing 67 tie down brackets, C7ZB-10578-79 -A for single exhaust cars, and C7ZB-10578-79 -B for dual exhaust cars. These drawings show a tie down bracket that looks very close to the ones used for 64-66 Mustangs as shown in my pictures.
For 1965 and 66 (both sides):
20430-S2 7/16-14 X 1 1/4L, GR 2, no head marking, 2 used into a welded in nut in the bottom of the frame rail. These are the same bolts used to attach the manual transmission case to the bell housing.
20546-S2 7/16-14 X 2 1/2L, GR 2, no head marking, 4 used thru the frame rail to a washer and nut. These are the same bolts used to attach the bell housing to the V-8 engine.
374358-S8 7/6-14 nut, flange 0.870 dia, stover lock, GR 2, 4 used with 20546-S2. These are the same nuts used on the bumper to retain the domed bolts.
(p/n unknown) 7/16 flw, 0.468id, 0.875od, 0.059-0.085t (as measured from several cars). I could find no other application for this washer in Ford documentation, however, it is identified in the AMK catalog for use on GM bumpers.
44730-S2 7/16 flw, 0.459-0.479id, 0.922od, 0.051-0.080t. I could find no other application for this washer in Ford documentation. It is a Ford standard washer.
Jim
