Was thinking of getting a miata open diff internal,welding it, and swapping it into my fc na diff carrier. Will I just need to swap my ring gear for it to bolt right in?

 

So the ring gear from my gxl diff fits on the miata welded diff but my stub shafts from the gxl diff doesnt fit. Would I need an s4 open diff stub shafts or miata stub shafts? Are there different sizes stub shafts? Is there a difference in size shafts between 1.6 and 1.8 miatas?