they are not "much lighter". Just a set of aluminum heads on the 440 makes them equal to a cast headed small block. And the aluminum water pump housing makes them lighter than a cast headed small block.
The weight of two iron heads (906’s) is 102 pounds without shafts and rockers and aluminum heads (eddy or source) is about 60 pounds, also without shafts and rockers. That’s a weight savings of about 40 pounds.
The weight difference on a RB vs La 360 I’d about 120 pounds.