I believe that the 351W's came from the factory with 9" rear ends. The 8" units were used on I6 and small (read lower power)V8's. Hence, my kids 69 SportsRoof with a 351W has a 9" unit and my 69 convertible with a 302 has an 8" rear end. That also explains the 9" in the early HiPo V8's.
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