Thanks for the information on idle speed. I suspect that is where the stumble might be coming from. I highly suspect your current idle speed is too high, which makes the throttle plates toward to upper end of the transition slot in the throttle bore.
While the specs on that cam don't seem too wild, for a (basically) 5 litre-size engine, they are wilder than they would be if in a 5.9L engine.
While I still had the orig 305 in my '77 Camaro, ti upgraded the cam to a "266 Energizer" Cam Dynamics cam (210 degrees @ .050" lift and .440" lift). I also installed a Holley 28-Z intake and an emissions-spec 4160 (1.56" throttle bore, 600cfm) carb. I had no issues with setting the idle at about 650rpm in "N". In "D", it went down a bit but was smooth. I never was really comfortable with the idle speed and mixture. I took it to work and hooked it up to the BIG Sunn machine to check afr and such. I never could get it as lean as Holley said was possible, as when I got the HC down to their specs, the HC figures went bananas, changing to wildly different readings as soon as the digital display could display them. Lean Misfire is what I was seeing.
The particular power valve in that carb was an emissions-oriented "two-stage" power valve. First stage came in at 10"Hg and the second (full) stage at the normal 5.5"Hg. Interestingly, in "D" with the a/c on, idle vac was 10.5"Hg. Just barely above when the power valve would start to open. When I did add just enough throttle in "D" to get it to start to open, it was not a big enrichment, but just a bit. Still, I wanted it closed in those situations.
So, I would suggest that you drop the hot base idle speed down to about 700rpm in "N"/"P" and see how things might change. Re-adjust the idle mixture screws at that speed, too. Plus verify the base ignition timing as you might also be getting a few degrees of advance at that earlier/higher idle speed, too. If your off-idle throttle issues are diminished, then we're on the right track.
IF things improve, then you might try my shadetree method of idle speed adjustment. Decrease the hot base idle speed until you can feel individual pulses at the end of the tail pipe, then add another 25rpm until they just smoooth out. This is with the car in "D" with the parking brake firmly applied, headlights on "Bright" and a/c, if equipped, operating. When things just smooth out, then optimize the idle mixture at where the idle speed goes to in "N"/"P". If that works as well as it did on one of our vehicles, it should result in a smooth and stable hot base idle speed.
Just some thoughts,
CBODY67