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I discovered my 64 has a C3 diff in it, the rear u joint for the pinion is 3.5 inches and the C2's are 3.25 inches. realized this chasing a vibration with a new drive shaft. By spinning the drive shaft and counting tire rotation it is somewhere around 3.30 to 3.50
I'll ask a question for the O.P.: Will a C3 Pinion Flange fit on a C2 differential (If it will fit, he still may have a C2 differential)
on my 64 the front bolts that mount the differential support should be threaded into the housing with the back bolt going thru the differential with a nut securing it as per the AIM. Mine has 2 thru bolts like my 71 has and my car was originally a power glide car that should have a 3.08 ratio, so I know it is not correct for my 64. Just trying to determine the year of the differential I have, not a big deal I bought a conversion u joint and all works fine. You never know what has been done to these cars over the last 50 years. With the Ls1/T56, Steeroids, disc brake and vintage air conversion the next owner should be totally confused . I am keeping a list of parts and putting it in the glove box