I think that a Corvair pitman arm would have the same spline as the Flaming-River steering box that you are using. I believe that the box you are using is designed after a reversed Corvair steering box. If you go to this website:
www.gasalleystreetrods.com/Steering.html. They show a number of pitman arms designed to use with a reversed Corvair type steering gear.
Good Luck
Don