There can be many differences. Some which may or may not be present:
(A) As a general rule, a carburetor for an automatic transmission is calibrated 1/2 size to 1 full size richer to compensate for losses in the fluid coupling. Hence lower fuel economy on vehicles with automatic transmissions.
(B) The presence of a "dashpot" (slow closing throttle control valve).
(C) The presence of a "hot idle compensation" valve.
(D) Vacuum ports for the distributor may be ported or non-ported.
(E) Throttle linkage may be different.
(F) A vacuum port may be present for transmission control.
These are general answers to a general question. Best to always know what characteristics are possessed by the correct carburtor, and then compare these to the characteristics of a different carburetor that you may wish to utilize.
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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