Here is the bracing on my gpz, quite applicable to CB750 too.
like Johnn, have created a support between the three top tubes, on top of that have also added bracing form near the top of the steering head tube to the two sides as well as bracing the corner.
Agree on tight fit bolts being a good idea, pierre des roches, the 70's American superbike tuner used to do just that, but quite a bit of work involved. at least in the rear mount that contrast most of the chain pull force, i did a similar solution to what is used now in modern sportbikes of a clamp fit over a large surface stepped bolt, off a Yamaha R1. It's also 12mm so I reamed the engine holes for a tight fit. on the chain side, i welded a support between the long spacer and the rear cross member to avoid bendign forces on the top bolt from the chain pull. My rear x-bracing was dome with alu 4 x 30 flat bars. as in an X brace its all about pull forces, i think this is lighter than round tubes and easier to produce.
Not exactly brace related but with the rear x- bracing adding so much stifness, you really need to get the free play of the swingarm right by shimming or a made to measure spacer. too much free play and you can really feel how you are stressing the frame trying to torque down the swingarm bolt. it doesnt make fitting the swingarm easy though, i actually expand the frame with a car jack to help!
That said, i've seen so many non braced frames in the winners cycle that i dont think any of this is top priority. Much more important IMHO is to get the set of suspension and geometry right.
unsurprisingly, most modern sport bikes have a threaded adjuster to take up play there.
