Due to a toploader transmission shortage, Ford also installed T-10 transmissions in V8 Mustangs for 1966 only, except HP. In 1965, the T-10 was not used on V-8 Mustangs, all toploaders. I don't know about 64.
Jim
+1. Midlife (1966 GT fastback from NJ in early January) came with a T-10, and 10 years ago, I swapped it out with a correct toploader, never looked back.
BTW, the toploader came from Mustang Masters (Fast Freddy) at a reasonable price.