Looks like this is an old thread, but in case someone reads it looking for this info...
The information I have shows that the Wide Grille Surround Molding (M28) was not available with Elastomeric Bumpers (M71 or M73).
Whether or not it was because they interfered with each other is not specified.
If the guy on the assembly line could not build the car as specified, who knows what he would do? You could have gotten one, or the other, or neither? I doubt they would have pulled the car from the assembly line for something like this, so odds are you probably would have gotten neither, as this is the easiest way to rectify the conflict of parts and keep the car moving down the assembly line.