I suspect the reasons Ma Mopar installed those brackets on the small block auto trans car was for resistance from breakage. A small block trans is only supported at the top 1/2 of the transmission. The extra brackets were installed on the bottom of the trans bell, giving support all around the bell. With the motor basically supported at the front of the motor, and the trans supported at the back of the trans, the bell to block is the weakest point in the support system, with breakage of the bell the result of a failure. I could see under grueling stress, the lack of support at the bottom of the bell could have been an issue. Since Mopar had the cover all the possible problems, adding the brackets to the small block auto trans was probably a cheap response.
The stick bell is stronger and more importantly, a separate piece from the trans. Should a failure occur, the stick would only require a bell replacement. A failure with an auto trans would result in replacing a complete trans case and swapping the guts into a new case. Gene