35-spline is 1.5"
40-spline is 1.708"
Set 20 bearings are 1.562" ID. (OD is 3.15")
Therefore, stock bearings won't fit a 40-spline axle shaft unless the shaft is machined down to 35-spline diameter (but then why run 40-spline?).
So to use a 40-spline shaft you'll need a 3.25" bearing case, big bearing ends (3.15") and you'll have to use a ball-type bearing instead of the tapered roller bearing (like Set 20). The drawback to the ball-type bearing is that it's great for the straight line guys, but bad for anything that takes a side load.