340 idles at 1500 won't stay running in gear

This sounds exactly like what happened to me in my Duster. After taking carb off and sending back to the shop that rebuilt it, I took it out and noticed a lot valve noise. Tried to adjust and got to number 5 and I couldn't adjust it. The ball was gone off the push rod. Got it out and, decided to do a cam swap. While removing the intake I found the two center bolt on the passenger side were finger loose. I'm pretty sure that was the cause for it running like yours is now.