Sooooooooo--------yes you can add the 40 gr to the crank in an area equalvent to the center of the throw-----or you can balance the crank with a bob weight on the throw that the rod will be with 40g lighter-------

Butttttttttt--------don't weld on alum rod-40gr is a lot of alum---------you might be able to profile the shape of the other 7 to lighten them

But I wouldn't run alum rods on the street------why not get some new steel rods that are BBC size, ford width and whatever length you need with the pin size you want????? And be cautious with the bearings when you cut them down to clear any radius on the journal plus rod side clearance----