Is it possible (realistically speaking) to replace a 4speed auto in a 92 400e with something like a 5spd auto from an S420?

Along with the rear diff swap, this would seem to be a logical way to increase low speed acceleration along with keeping freeway revs down.

Let's assume for a moment a whole donor car could be sourced for all of the needed parts or is there something totally obvious that I am missing?

At this point, this is more idle curiosity than anything else.

Thanks,
Jim