If you can get a socket on the bottom bolt under the pinion then its an 8 inch.If only an open end wrench or box end will go on it then its a 9 inch. 63/64 Galaxies had 9 inch rearends. The mid size Fairlane/Torino platform with the small block would have had the 8 inch.
After 25 years with condensation in it they may be rust pitted.Won't know until you pull a chunk out of your rearend .
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