518 short shifts and no od

Learn to adjust the kickdown-linkage properly. Wrongly adjusted, it can kill the transmission in very short order.

To get a baseline adjustment, adjust the kickdown linkage so you can 'just' slide it over the pin at the throttle-linkage, when both are held at fullthrottle-position. (throttle fully open and kickdownlinkage pushed all the way to the rear).

Take the car for a drive and see if the shiftpoints are happening at 'normal' rpms.
Finetune the kickdownlinkage if necessary.

Then test the OD-function and re-adjust the shifterlinkage.

Do you have a carb on the car with original Mopar-linkage, or carb with a modified GM-linkage?