Don't rule out a vacuum leak if the issue persists. I chased an idle/low rpm issue for over a year . I couldnt find a vacuum leak so I focused on carb/dizzy tuning with mixed success .
Finally I ordered Eddie embosed gaskets and Gasketcinch adhesive just to see if it might help .
Voila ! My engine idles much better and way smoother below 2000 rpm .
The leak must have been below the intake.. I even tried propane to find leak .
This is a 5.9 with Airgap intake and 222/229 @.050 .510 lift cam