I may be able to shine some light as I owned and built a 13.5 for 5 years.
The part you show a picture of is needed. You can. UT it easily for West Covina nissan, unless the stopped selling g it, but when i did this, they had made you s15 items.
The part I did not see mentioned and a part I brought back with me from Okinawa is the part that you mount the bumper retainer to, which also supports the headlights. It is the actual s15 metal bumper. Where you can find it, not sure state side, but I am sure you can. I think I have a photo that may show it well.
Using both the solid bumper and bumper retainer align and support the bumper very well. The retainer helps enormously with the look and sagging as well as keeping the shape when working on it.
I can dig up more off my phone later.