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The TH350 trans requires an adapter spacer from Hughes (PN HP3795) and a flat flexplate (Hughes PN HP4004). This adapter is necessary to make up the 0.400-inch difference between a Gen I small-block crank flange and the shorter LS flange that is effectively even with the bellhousing face. The adapter mounts directly to the LS crank and then the flexplate bolts to the adapter with longer ARP fasteners. Because the new flexplate is flat (factory flexplates are concave), the ring gear teeth line up perfectly with the starter.