Is your Holley a mechanical or vacumn secondary? if it is a vacumn secondary then I suggest replacing the spring in the vacumn pot to about the third from lightest and shifting out of first around 62 to 65 and then 6000 for second and if you find yourself getting close to the 4th gear shift after the 300 meter mark then try holding it in 3rd as long as you are not going to carry it past about 6500 in 3rd. If the carb is mechanical make sure the linkage to the secondaries is working properly. To run high 12's with a trap speed under 160Km sounds like fuel or air as stated before. From what others have said and your responses my guess is you are running primarialy on 2 of the 4 barrels of the carb.
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A good plan, violently executed now, is better than a perfect plan next week.
George S. Patton, General (1885-1945)
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