318 LA rebuild after losing a valve seat

Are you saying?
that with BOTH shifters removed from the trans, That attempting to shift just the little short lever, is a no-go?; and the little lever is NOT hitting anything externally?

In that case , I suspect the trans is jammed in Park. That can happen if the E-brake is on, or the car is parked on a slope, and only the park-pawl is preventing it from rolling. Chock the tires, and raise one wheel off the floor, if an open diff (both if a SureGrip) and try again.