Download a copy of the Sress Analysis study that I did.
CB400F Stress Analysis.pdf
As I mention in the study, doing a good job of bracing the stock frame is impossible because of the Gas Tank occupying the same space as where you would like to put the frame bracing.
Here are some photos of a braced frame where they made a small aluminum fuel tank to fit around the bracing, and then used a stock outer tank skin as a cover.
You can just make out the perimeter tubing flanking the backbone to the lover part of the steering head, and another set angling up to the top of the steering head.
I believe the running gear is NSR250.
Here you can see the upper tubes curving in to meet the top of the steering head.