Installing steering shaft...

With the steering gear centered straight ahead the master spline will be on the top. The coupler has a tiny notch it the outer leading edge relative to the naster spline location inside.
If you got it started on at all the master spline is probably right.
You're not the first to fight this coupling. Look at recent threads for comments about how the coupler often binds at cutaway for the cross pin and how to make it easier.