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At that altitude, you'd want to go down 2 jet-sizes to compensate for thinner air. Most likely not your issue as richer-than-ideal mixture won't cause it to not redline.
Most likely you have an insufficient fuel issue due to clogged fuel-circuits. Or clogged vacuum circuits leading to slides not opening enough. Are you able to disassemble carbs down to last nut and bolt to clean?
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