Good deal. Always good to go with hardened in any case. I like Novak's, good quality. Be careful when you change you're spur, that you don't spill to much of the dif fluid unless you have more to top it off. If you're changing from plastic to steel, you will need to remove the plate under the plastic spur gear.
When you pull off the dif spur side, put it spider gear side down on a towel and the spider gear will slide off the output shaft on it own. You won't usually get a gasket with the new metal spur, so you'll have to reuse the old one if it's good or replace it if it's torn.

Reassemble and make sure the mesh inside the dif feels smooth. Remesh the mtr and pinion and go try it.