The engine really can't be considered a stressed member without ream fitting larger mounting bolts. The original bolts are just too sloppy. Both turboguzzi designed a "head steady" for his CB500 racer and friends do the same with their CR750. However it's the pressed steel surround of the steering head that would be best supported and that isn't easily done.
The swingarm pivot area is another spot that is often stiffened. Originally I used a bolt in alloy brace here using high grade splined bolts plus welded braces underneath.
As far as welding in braces. They should be welded on the outside of the tube not the middle, and a saddle style brace is best. Note where the welds start & end, don't weld solid.
Where Honda joined the front downtubes they made three spot welds. That area can be welded down the joints.
Kemp's bolt in brace in the front is a good idea.
Most often all we're doing is adding weight as the frames are quite adequate as is. The Italian 500 class champ that races a CB500 has no frame bracing at all. Makes you wonder!
Hope you don't mind Don.
