Heres some food for thought. I have an AOD car with more mods than you, and if I run it at the track shifting manually at around 5300-5500, or if i leave it in Drive and let it shift itself at around 4000-4200 rpm, my e.t. is exactly the same. These motors make power a lot lower than people think. Your rpm reading was correct.
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1993 GT/AOD
'93 Mustang GT