I have a 80 Mustang I6 and a 81 Mustang T-top coupe w/ I6.
It does require a I6 K-member. The motor mounts are way forward and the K-Member is a bit deeper.
The I6 was offered in 79 1/2 to 82 Mustang's. There are 4 speed I6 cars but most are C4 auto's and the last group, IIRC 82's also...