Reinstall the linkage arm. While pushing the arm back onto the lever coming vertically out of the transmission, the lever may rotate which changes the gears in the transmission. Rotating the lever all the way counter-clockwise will put the transmission in Park (this is where it should have been when you removed the linkage arm). Secure the linkage arm by tightening the bolt.
The hole at the end of the shifter arm should line up with the hole in the linkage arm.
Insert the 3/8 grade 8 bolt through the hole in the linkage arm, slide a washer over the bolt, slide the bolt with the washer on through the hole in the shifter arm, slide the last washer on the bolt and thread the lock nut in place. Tighten, but do not tighten all the way. You want it to be a little loose so the shift arm can move back and forth freely.
Move the transmission shifter in the cab through all the gears and make sure they engage properly and the shift is smooth. If not, you may need to tighten/loosen the bolt. If the vehicle is not in the gear specified on the transmission shifter, you should check to make sure you had the transmission's lever in the correct location when you reinstalled the linkage arm.
Congratulations, you're done. Your linkage arm should look like that pictured, and you can rest easy knowing that you have a solid grade 8 bolt holding your linkage together rather than a cheap plastic bushing :)
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