the bumper are ALMOST the same. the only difference is on 72 there is a slot cut out for the jack. 71 uses a hook so it doesn't have a slot.
i have a 71 bumper on my 72 road runner now. the grille and brackets bolted up without any problems. when i bolted it on i didn't even notice it was a 71 bumper until i was checking it for straightness. after 5 years of going to shows maybe only 3 people have ever noticed it.