I did this on a '75 Dart Sport and it moved back enough (fully collapsing the bumper shocks) to look like it was factory. Sorry, I don't have any pics of the car and it's long gone. I currently have a '75 Duster and will probably do this in the future, but my filler pieces are like new, so it's not a high priority at this time.
My filler is in very nice shape as well. I'd think if you took it out and couldn't get the bumper back far enough a gap between the bumper and valance wouldn't look right.
Did you remove any of the reinforcement behind the bumper?
