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Easiest way: Raise car 18 inches, disconnect shifter rod to 1-2 (middle) lever by pulling clip (or cotter pin), make sure both shift levers in the cover are on the neutral detent, set shifter in neutral on 3-4 gate making sure also lines up with neutral on 1-2 gate (a helper here is best), and reinsert the 1-2 gate rod in the lever hole while adjusting the length of the rod by turning it in the swivel (clevis or pin) end.
Using pins or plates in shifter slots and holes does not always produce the desired result in old worn shifters. Badly worn or missing bushings at the ends of the rods should be replaced. Cotter pins holding shift rods in the levers should be discarded in favor of GM spring clips.
If I had to go to all the trouble to get under the car, I would bolt a Hurst shifter in there and put that junk stock shifter in the round file where it belongs.