Update: I narrowed the problem to the idle circuit in the Edelbrock 1413. I could turn both idle screws clockwise all the way shut and the engine would speed up.
I took the top of the carb off and cleaned out the idle bleeds, which were clogged. The discharge nozzles were also partly clogged. I ensured the float level was 7/16". I put it back together and then I ran the car and sprayed lots of carb cleaner in the idle screw holes with the engine running. The stumble is gone and it runs better, HOWEVER...it still can idle with both idle mixture screws closed. Now I get the highest idle vacuum with the drivers side idle screw only 1/4 turn out, and the passengers side about a 1/2 turn out.
That's still nowhere near the 1 1/2 turns it should be, so...I still have an idle circuit problem somewhere.
What should be my next steps?