Can you find out what kit was installed (if it was a kit) or what mods were made to the valve body?
It sounds like there is a problem in manual selection of the gears. if it shifts ok in drive then drive it there and when racing you can shift manually. It's not a fix but it does by-pass the problem until you can sort it out.

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Paul
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retired mechanic after 35 years
specialized in Holley carbs and Ford Automatic Transmissions