This upside down view of a side oiler engine shows the bulge on the left side of the bottom of the block.  The bolts for the cross-bolted main bearings are also visible just above the pan flange.  All 427 engines, including the center oiler, had the cross-bolted main bearings.

The 428 engine is flat to concave in this area, matching the other side.

Side oiler engine

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