I assure you, all Fox bodies that had door speakers from 84+ have 6.5" drivers in the doors. Mustang, T-bird, Cougar, Mark, LTD, etc. . .
As for the rears, you can find drivers that are marketed as 5x7/6x8. These are essentially a 5x7 with additional mounting holes that fit 6x8 mounts. Some are slightly larger so they will not fit a traditional 5x7 opening.
I am pretty sure stock (from the factory) were indeed 5 1/4" door speakers. Many people replaced them with 6 1/2" speakers and I believe you can get away with putting in 2 or maybe 3 screws.
I wouldn't call it correct installations, but many people have different definitions of correct AND installed