Need help on wheel studs

[bugs4121]

I need some guidance.....the rear wheel studs on my 72 k20 Eaton rear happen to be 1/2 and the fronts are 9/16 is this normal or did somebody cut a corner before I bought my truck???? I need to replace a few that are damaged , will the 9/16 fit into my rear hubs the same as the 1/2 studs and finally when the chrome lugnut says 9/16 X 18 what does the 18 stand for ????

Thanks