I have only driven my 496 a couple of times since my car is still incomplete but it runs great!! I have a Holley 850 double pumper on this engine. It is so snappy at operating temperature, no stumble. All we changed was the squirters for the primary accelerator pump circuit. Other than that just the jets and idle circuit. Holley's are pretty easy to tinker with. I have no choke on it and have to hold the throttle open a little when it's cold but after a minute it's fine.
Lean conditions under throttle transition on a double pumper happen from improper accelerator pump set up, usually just the squirters, or the wrong power valve. Distributor advance is also very important here.
Nothing wrong with vacuum secondary carbs.... There is a reason the factory used them, they're much smoother, economical, and easier to tune. Usually easier to get the cars to hook up on street tires with a vacuum secondary too, but with a 496 I think the chances of you getting traction on the street is probably near 0%
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