Which side cover to use

The double lever interlock side cover assembly was introduced in late 1970 on the AAR 'Cuda and T/A Challenger and was used on all transmissions from 1971-74. The older ball and detent unit was used on all 4-speeds prior to the aforementioned change. The newer double-lever interlock unit is interchangeable with the older units and vice versa if all pieces of the side cover assembly and shifting mechanisms are transferred. This situation is useful since drag racers generally prefer the earlier version and street drivers the newer style.

http://www.mothermopar.com/4sp_transmission.htm