I got mine as a combined kit.
The swing-arm fills in the space in the middle of the bumper.
I don't know what you get if you order the lower part only. I don't know if there's a filler piece to cover it up.
However - the ARB rear Bumper is much higher clearance than it looks.
Take a look at the picture with the swing arm open. Look how close the tailgate is to the top of the bumper. This is because the bumper is covering up the bottom of the tub, instead of being below the tub like stock.
This means using the stock tire mount, you've lost several inches of clearance between the tire and the bumper.
Save up a little longer and do it all at once