40 spline is a waste, 35 pretty much standard across the board. Aluminum is strong enough to get the job done and then some. If you need weight back there ,go with the iron, If not and the price is OK then the alu. Spline ingagment should be equal to the dia of the axle plus 25%. Be careful on mixing supliers, MW offers a propiatary spline along with standard spline.
If your housing has Ford bearing ends and is 27-3/8" bearing flange to bearing flange, I have a really nice pair of standard 35 spline MW liteweight axles w/studs and bearings