I remember that well, I think Ford tagged it as a 64 1/2 Mustang, the 65 came out a short time later in 65 if I remember right.
I nearly bought a 65 but they wouldn’t knock a dime off the sticker price so I walked, wish I had it now.
My Brother had a 69 Mach 1. The styling had changed just a little but it was a nice looking car. It had a 351 with a four barrel. I don’t remember the HP but it was not as hot as I thought it should have been but not slow either.
It did sound real good.