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If the kickdown-linkage isn't setup properly, the car will either upshift very quickly while taking off, or it will hang in gears longer than necessary.
The most common issue is some folks don't hook up the linkage at all because they think it's not needed. But on a Mopar transmission this linkage is very important because it controls the oil pressure inside the transmission.

Without the linkage, the oil pressure will be too low during acceleration and the transmisison will slip internally, making it wear out very quickly.

A guideline for a basic-adjustment the linkage is that when the carb/throttle is fully open, the kickdown-linkage should also be at it's most rearward position.
From that point the linkage can be adjusted a few turns loose or tighter to have the transmission upshift at the correct time.
It will also control when the transmission will downshift into a lower gear when the gaspedal is mashed.