48 is right;it's the old "weakest Link principle. By using 3/4 ton springs,you make your rear axle bearings,and the axles themselves,the weakest link. If you then swapped in a 3/4 ton rear end,that could move the weak link to being the frame,or u-joint,or control arms...I'm sure you get the drift. You just need to consider the "what would go next" thought process when you upgrade or beef a component up.
Speed
BTW-I'd have gone with the overload springs-they don't affect your ride until you load the truck down.
