Is the exhaust routed near the starter motor? Usually when this happens, it's due to an old stater motor that functions poorly when it's hot. Even a new starter cranks slower if it's hot. At least, that's what I've always seen.
Rev
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'66 Coupe, 306, 300 HP, C-4, 13.97 e.t., 100.3 mph
1/4 mi.
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