I have only done it when the body is on the chassis, but it is a bit of a pain to get it into place. I agree that doing it on the chassis is best for alignment purposes, so if you have the option, I'd suggest that you take advantage of the body being off the chassis and actually do it by clamping the crossmember onto the body, then lowering it into place on the chassis before welding it into place. That gives you the advantage of plenty of space to manoeuvre the crossmember into place.