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You do not mean the housing? The 66 is an 8inch and the 90 is a 8.8. There was a 7 1/2 in the family somewhere, but I think it was on V-6's They do make kits to do a 8.8 to a classic platform, which is really inexpensive if you compare to the 9 inch. You can pick up an SN-95 with disc brakes and save money on a rear disc swap.

I have heard the best route to go is using the explorer rear end.You end up with an 8.8 that usually has the desirable gears plus it's a 31 spline