The 1956 and earlier three bolt two barrel carburetors are specific to their factory supplied1956 and earlier Load-O-Matic distributors .  If hooking up the earlier carbs with the 1957 and up distributors, drivability issues typically come to the forefront.  Because the earlier carbs use both ported and venturi vacuum in a shared circuitry within the carb for the vacuum signal to the distributor, too much vacuum is typically available during cruise situations when using the earlier carbs with the later model distributor which in turn makes too much ignition advance available.

Here are modifications for converting the pre 1957 Holley three bolt (Load-O-Matic) two barrel carbs to work with the 1957 and later distributors.  Not the only way to do this but effective just the same.