will 70-72 bumpers bolt to a 73?

The mounting holes in the later frame rails were moved. To use the early bumper braces you will need to drill holes. Nuts would need to be fabricated from square blocks of steel that wouldn't spin inside the rail. A length of coat hanger wire tacked on the side of the nut is also needed to place it behind the hole from the front end of the rail.
Hope this helps.