I knew the air filter housings where not the same 260 vs 289. I do not know why the filters would be different
There is no difference in the housings either. This was somehow perpetuated in the hobby for a while, but we learned at least 10, if not 15 years ago that this simply was not true and the MCA rules were changed.
If you ever see one of those big Falcon/Fairlane air cleaners on a Mustang, look at the snorkel and you'll see a rub mark where it makes contact with the bottom of the hood. Physically incompatible with a Mustang.