This afternoon, I tried Martin's idea of mounting the brackets on the bumper first and then see where the brackets fit the frame rails.
The brackets ended up being close enough that I could start the bolts and then tighten them down to pull the brackets into position on the bumper.
The splash apron (CounterPart) on the other hand took a little bit of work. Apparently the two tabs on the left side were not welded to the apron in the correct positions, they were about a half inch too far toward center. This resulted in the slotted holes not allowing the apron to center with the bumper. Luckily the tabs were wide enough that I could drill a hole and use the jig saw to cut the slot further in the proper direction.
The brackets, apron and bumper then bolted together as they should.
I did not have any help today, so I could not mount the bumper on the truck. Measurements indicate the brackets will fit snugly on the outside of the frame rails.
