Stumble issue

Did you blow the carb orfices out with carb cleaner and compressed air ?
One spec of dirt in the wrong place can dog any carb.

Stumble from a complete stop usuaslly is related to the pump.Might be you need to replace yours.They are available from Edelbrock.

Are you getting a good shot of gas when you hit the throttle,and is it instant ?

How did you measure your floats ?
I usually set mine a tad higher than what they ask for.