For going SOA, guys will generally re-use their current leafs. All depends on the condition of the leafs and what your overall plans are for the truck.
"Splines" (in this case) refers to the number of little ridges that the axle shaft has that fit into the spider gears inside the axle. As a general rule of thumb, the greater the number of splines, the stronger the shaft is (the exact specifics as to why are complicated). All your splein counts match up because you're buying a complete axle. don't' worry about it for now. Later on, if you wanted to get an overall stronger axle, you could get some 29 spline shafts and spider gears from a different junkyard 8.25 and swap them into your new axle.