As a general rule (not just Q-Jets), carburetors designed for use with auto transmission vehicles are calibrated about a half a size richer than those designed for standard transmissions. The extra fuel is necessary for the additional power loss in the torque converter.
We tell folks a carb designed for an automatic may be used on a vehicle with a stick with a few percent loss in fuel economy; but using a stick carb on an automatic will generally require mild calibration changes.
Also, the vacuum port differences mentioned by others in this thread.
Jon
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"Good carburetion is fuelish hot air". 
"The most expensive carburetor is the wrong one given to you by your neighbor".
If you truly believe that "one size fits all" try walking a mile in your spouse's shoes!
Owner of The Carburetor Shop, LLC (of Missouri).
Current caretaker of the remains of Stromberg Caburetor, and custodian of the existing Carter and Kingston carburetor drawings.
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