340 dart shifter help needed!

If the threads are gone like you say you are gonna have to drop the side cover and replace these pieces. Make sure they have the correct fine thread nuts, and not rigged up with more than one lock washer. Give the shifter and rods a good look over. Tight bushings and spring clips on the rod ends are a must. Give the shifter a good adjustment, including stop bolts if your shifter has them and you should be good. Check with Brewers for parts.