With a .060 overbore and stock cylinder heads and valves (gasket matched to 340 intake) we ran 12.72 and later with 340 heads (2.02 intake valves and notched block) I never broke out of the 13.00's.
At the time (1970's) I was perplexed, but now I believe the lesser performance with the 340 cylinder heads was due to lower compression because of the larger combustion chambers.
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