1984 GT Mustang Body Parts?
Check with local racing enthusiasts, and local mod-shops, car shops...etc. A lot of the time you'll find guys that have spare cars, etc.
Also be sure to hunt around wreckage yards. Fox Body Stangs are known to get wrapped around poles...A LOT. You may get lucky and find a few.
We own two '83 Stangs, and we got really lucky with a few parts. Our first '83 is an original 5.0. We're getting collector plates for it, and therefor, everything had to be an original part. It's a black interior, and you wouldn't believe how rare an original black interior is.
We struck lucky on a bunch of parts...but only because my husband has hook-ups. LOL
Good luck in your search!