It looks like the left downtube showed signs of failing prior to the failure, the right side looks like a fresh failure. This would be another argument for Sandblasters front head stay, had you had one it may have prevented complete failure. Welds appear to have held. I do believe that the factories use the radius tubing to prevent failures like this. Seems like an old welder told me one time that you never weld square, always radius you pieces to prevent failure.
This was a 250 so, all with 500's better look close, really close.