Hi Ken
Pretty sure the 460 is 18in, bunnings special
If your mower shop can't find/doesn't know what blades fit that model, change shop
GA spares BNC5345 for blades and bolts
With regard to the disc cracking, I have always been convinced it was through neglect and hitting stuff. But, I also agree accidents happen and its a good idea to strengthen it
BLH7934 for the blade stiffener
regards