The 73 bumper is the same as the earlier years. The 73 used different style bumper guards then the previous years. The other difference will be unless you change them, in 1973 to meet new federal regulations Ford used different bumper brackets which made the bumper stick out some away from the body of the car.
brackets will the 73 bumper sit closer to the body if I use the 70 brackets? can I use the 73 with the 70 set up?
That would work. I need to find a set of 70-72 rear bumper brackets to bring the bumper on my 73 closer to the body.