what year 300zx? i figure you bought an n/a z32 (90-96) diff? 5 bolt axles you'll need yes, only have to swap the cover if it's going on s13. s14 and z32 are same covers, just z32 has cooling fins.

i forget if the z32 axles will work in the s13 or not. it will depend on the spline size mostly. you could try j30 axles, they are 5 bolt as well. you can use your stock 6 bolt axles, but you would have to swap out the output shafts for 6 bolt units.