Just spitballing here- but if the passenger side is OK, then that part of the setup is good. No mod to the motor arm as that would effect both sides. A mod to driver side arm would seem likely. Shortening that arm would increase rotational travel as the connecting rod still travels the same amount. I would guess the arm or the connecting pin would not need to be moved very much. You are already getting 60 percent, so maybe 1/4" or less might give you the extra 40.
