1970 used a different mounting setup than 71-74. Your 71 should just have one bracket in the middle of the valence.

Also, your going to have to remove that triangular lisense plate bracket. I can see it in your picture. That bracket was used in 73-74 only. In those years, the rear bumper was held away from the body farther than 70-72. The lisense plate needed to be held out farther because of it.