Equal length headers to 340 manifolds. Re jet carb?

This;

Best time with slicks, headers and 3" exhaust 11.78 @ 118, best time with manifolds, 2.5" exhaust and E70-14 Redlines 13.2 @ 105

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Lead me to believe you went from manifolds to headers which would, of course cause a lean condition.
Of course, the title sez differently. Duh on me.
Do you have an air/fuel gauge on hand?