Gen 1 is Falcon based platform. I hear you on the body changes for the 71-73 really not making one feel that they all belong in the same club.
Gen 2 is Pinto platform.
Gen 3 & 4 is Fox body ( and then modified Fox body renamed SN-95) was originally a shared platform but Mustang was the last to use it.
Gen 5, 6 and probably beyond the platform is Mustang exclusive, at least until everything is one EV rollerskate fits all...
Pinto came out in 71. It was a rushed development program with a much shorter lead time then the industry norms at the time.
Mustang II was evolved out of this program. It came out in 1974.
I agree they are a Mustang. I have defended them on these forums quite a few times actually. The Pinto platform thing is basically just a trivia question, though it never hurts to know what vehicles share components with one you might have.
It's Pinto derived. Dick Nesbitt's design team was originally going to base it off the Maverick but new Ford president Lee Iacocca wanted it smaller.
No you can't just slice a Pinto top to bottom and plunk a Mustang II onto it. The unibodies were different with the key being the isolated front suspension subframe --aka the thing hot rodders have loved about the II. Many components were the same though, and the development costs were shared.
Pinto has some lineage back to the Euro Escorts of the day too.