holley carb help,transition/stage

the transition slot is mostly affected by your idle feed restrictor, i would go up .002 or .004" in size and see if that cleans it up at 2000rpm steady throttle.
the stumble, make sure your accel pump linkage is tight against the arm (messing with idle speed screw can cause changes in that adjustment)...if that's good, maybe look at your pump cams next.