[scicala]

There are actually three different thicknesses of accel pump covers. Your's pictured is '57 only (first year of the 4100). The second carb pictured has a pump cover used from '58 thru approx. '63 or '64 (shorter than the '57 style). The third style from '64 and up is the shortest of the three styles.
Shouldn't be a problem converting to either of the other shorter styles. You will need the newer accel pump diaphragm (#59) available at auto parts stores, and a shorter pump rod that goes from the throttle lever to the pump lever. The return spring (#60) should be useable from the early carb.
Of course you will also need to find the shorter diaphragm cover of your choice.

Or get a later 4100 if you want more airflow. The '57 Autolite 4100 has tiny ventui's and may flow about 400 CFM.

Sal