I think you'll find that the wiring for the rear of the car to accommodate the fastback will be a major task due to the 84's unique wiring and grounding. If you could find one, upgrading to an 85 or 86 4-cylinder and dropping in a 5-speed Isuzu transmission would vastly improve the reliability, streetability and economy.
I don't know that the Indy's ever came with an econo 3:32 transmission, so with the standard 4:10 transmission, your maximum speed will be about 60 mph. At that speed you are running the engine up close to 4000 rpm. The 5-speed would drop it down below 3000 at 70 mph.