I usually don't post very little thing I do on forums. I post a little text and some pictures and let you guys fill in the blanks with the hope that you guys will just assume the fabrication is good. Clearly some of you have more time on your hands and need more hand holding than I would generally provide.
Here is a picture I colored on to show what I feel are the load paths, dynamic and equalizing. It shows gussets and load distributing triangles. It shows the dynamic load (green arrows) going up the frame from the start point of the load at the rear tire assuming the pivot point of the load would be the location of the stock swing arm support. It also shows the equalizing load path (orange arrows) that will travel through the gussets and supports to distribute the load and prevent failure. In the picture below "A" is assumed to be the failure point if failure was to occur. What you can't see and what motofiaccone assumed wasn't there is a plate that runs above and below the point of attachment to distribute the load. Not that the load at point A would be that significant since the load is being distributed along the path of the orange arrows.
That backbone? .090" sheet metal which is trying to buckle in it's stock form.
The backbone has sheet metal in the construction (area "B" above) of it but unlike a stock CB350 there is a 1.25" steel tube (3/16" wall thickness) that is welded into the neck and is fully welded through the remaining stock lower backbone (where the top engine mount is) and then welded to the down tube at the rear of the engine. There is also extra support connecting the pipe to the neck that was not shown on this thread. The sheet metal piece I welded in is 16 gauge (approx. 1/16" or 0.06" thick which it comparable or better than stock) and is welded to the neck and along the backbone that I put in. The angle of the triangle that the sheet metal makes is appropriate to distribute the load along the backbone of the bike. If the angle was too steep it would not provide enough rigidity to support the load. The shape made is almost a perfect geometrical triangle.
A 250lbs man just sitting on the bike could do the trick.
This thing isn't cardboard, its steel. Do you really think a 250lbs man just sitting on the bike would cause the frame to break? Seriously? I'm only 170lbs so I guess I'm good to go right?
Feel free to get your crayons out and color on a picture of the bike if you want but ultimately without seeing what I have done in person and the steps I have taken to ensure the rigidity of the frame you have no sold platform to base your unfounded opinions on now do you?