Replying to Quick question for someone who knows about longboards (wheels)

I am about to buy a new set of wheels for my longboard. before I do this i need to know one thing. Is the rod thing that comes out of the side of the truck which you put the wheel on a universal length? or should i measure and look for the spec on a wheel. the only specs on wheels i could find are; 70mm (i would assume is just the width of the wheel) and 80a (a measurement of how soft the wheel is).

also. I use my long board down pretty steep hils on fairly smooth pavement. I would like it to grip well but also allow for some sliding and drifting at moderate to high speeds. Is 80a fine for what i am looking for or should i go softer? too like 78 or 79

plz help!

+k for anyone who responds with something helpful