To answer several inquiries; the knob does fit
tightly on the original Fiero shifter and it will shift into gear with the original Fiero plastic rod insert;
BUT YOU WILL HAVE TO FIGURE OUT A WAY TO HOLD IT STEADY AS IT WILL SLIGHTLY MOVE UP AND DOWN WHEN YOU DEPRESS THE SHIFT BUTTON You may have to drill and tap set screws at the base of the knob or modify the existing screw hole to hold a set screw or drill and tap the original Fiero shifter in order to thread the original metric screw that comes with the shift knob.
See new pictures below.
[This message has been edited by Chief (edited 08-05-2015).]