If I understand correctly, after my research, the T5 transmissions found in the Camaro 84 come from the Mustang manufactured by the Ford company?

 

The T5 was manufactured by Borg Warner. There are several different versions designed to fit different applications. Put a Ford T5 next to a GM T5 from a Camaro, and a GM T5 from an S10 pickup and you'll see three very different transmissions.

 

In '84, the Mustang still had the good V8 ratios.
So in that year, the mainshaft, maindrive, case and extension housing were different but some gears interchanged between the two boxes.
Borg Warner's light duty division was sold to Tremec in the 90s. So they were a Tier 1 supplier to GM and Ford in '84 for manual transmissions.