Wrong again. It was in the 1971 Daytona 500, the first race of the season. The Hemi wasn't banned, it (and the 427 SOHC) were just banned from aero cars. Most teams reverted to non-aero standard B bodies and continued running the Hemi. As a matter of fact, Petty won the championship that year with his Hemi Roadrunner.
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He repeated in '72 (but switched to a Dodge Charger mid-season)
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And won the 1973 Daytona 500 (but lost the Championship to Benny Parsons)
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All this was after the little 305 had proven it's mettle in the 1971 Daytona race.