Looking at the bumper diagram below Items 2,4,5,6 are to mount the bumper to the frame(bar) bracket and the bumper stabilizer (support) bracket to the frame bracket.
AMK kit G-7424 for this, is almost correct.
The flat washers in the kit are to thin. AMK does have the correct washers (Pontiac #531315, AMK B-11286) however.
diagram from the Restoration Guide