You can tell by looking at the inspection cover. If the top and bottom is flat and the sides are round it is a 7.5". If it has four flat sides it is an 8.8". If you have a Summit or Jeg's catalog laying around, look in the axle section and it should have chart showing the gasket profiles. Ford didn't start putting the 8.8" in the 5.0L Mustangs until '86. If your car is an '83 GT it will have a 7.5" unless someone has already replaced it.
I was thinking it was a 7.5 rear wasn't positive though I'm looking at throwing in 3.73s or 4.10s as compared to the 3.08 I have in there now not too concerned with the rear itself mine is in really good shape even if its a 7.5 it only has 40K on it...a full rear swap will be later down the line thanks for the help
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