Getting a 12-valve to idle at the correct engine speed is one thing; getting it to stay there over its lifetime is another. Late model 12-valve Dodges should idle at 750 to 800 rpm, in drive with the air conditioner on with an automatic transmission or in neutral with the air conditioner on with a manual transmission. However, the factory gauge isn't entirely accurate, so you may need to go by sound to get it where it should be.
Much higher than 800 rpm and the truck will have too much of a walk away force in drive with your foot on the brake. Much lower and it may stall in colder temps.
To adjust the idle, manually depress the throttle cable lever on the p-pump (with the engine off) and loosen the 10 mm lock nut on the idle speed adjusting bolt. The idle speed adjusting bolt sits near the rear of the p-pump and faces upward. If you follow the throttle cable coming out of the firewall to the p-pump, the idle screw is about the first part on the bottom side of the p-pump that the cable crosses over. You can reach it by going between the brake master cylinder and the p-pump. Once the lock nut is loose, back the screw out to lower idle rpm and turn the screw in to increase idle rpm. Tighten the lock nut when finished.