Steve peak horse is 6800 so optimum shift point would be a little over 7000 but using the throttle rod to restrict the opening up of the carb also limits CFM so trying to spin it to 7000 on what is effectively a 500 CFM carb does not work. I usually shift at about 6300 but when the air is poor it struggles to get up there in rpm. Yesterday it was 2800 feet here and 50% humidity so when I tried shifting at 6000 it actually liked it better than waiting for the rpm to hit 6300.
1968 1/2 428 CobraJet Mustang
2009 Mustang GT
38 foot Diesel pusher motorhome
26 foot Pace American trailer
plus 3 other Fords