First: snafus. In determining the length of the headboard, I measured the bed. I should have measured the metal frame the bed sits on. The difference is about 3/4 inch, but it meant that the headboard wasn't wrapping around the frame to allow the horizontal board to sit flush against the frame.
To address that, I started by tracing around the frame and chiseling out arcs. I used a chisel and hammer, dremel sander, and finally a mix of the oscillating saw with the other two to achieve the effect. The intent was for the frame to then wrap snugly around the metal frame.
Apologies for the terrible pictures. This room is in the front of the house, meaning it gets little sun, and it was the evening, so even less sun.
After the headboard was hugging the metal frame, I traced where the bolts would thread. The bottom rectangle is below the frame that the box spring sits on, so that was the best option. The top rectangle would mean the end of the bolt would poke and scrape the fabric. I then drilled holes in as straight of a line as I could manage (and still had some fixing to do).
My assistant, Henry, was on hand to pass the bolts.
With a pair of pliers and a socket wrench, the bolts were tightly secured. Again, with them under the frame, they won't tear up with mattress/box spring, and they're hidden in the corner bracket, so legs are safe, too.
That was boring. It's gratifying though because for as quick as the headboard came together, attaching it took days. It was wrong measurements of bolts, wrong measurements of frame width, wrong everything. Now it's done.