[elefantrider]

I agree the stumble is a combination of the aluminum flywheel and the lean condition.

I would have thought a '71 455 would draw need more fuel at idle than my 433ci.
61 main jets are certainly much smaller on this big carb than was stock for tripowers center carbs. Probably due to increasing emissions regulations in the '70s.
I believe they are not in use when the throttle is less than 1/8 to 1/4 open, so probably need to open up the idle tubes. I will look for the equivalent size drill bits.

There is no choke currently hooked up.

I will post a picture to see how a choke could be made to work. I'd need a '66 choke stove assembly , and could somehow rig up the carb to that.