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I usually shifted mine at 7500, after that I really didn't gain anything by turning it higher. I had stock heads on mine, GT40 heads or maybe some of the newer stuff would have flowed better.
back when i had a 71 mach1 with a 351 i raced my step dads 55 belair with a dz302 he up and left me lol never count out the old timers they are faster then you think