I welded the forward downtubes on my 750, same as you. I opted out of adding any extra tubing or bracing as the only spots available that wouldn't interfere with wheel travel or the engine were well braced already. I know people have added more, possibly at the steering neck, but what was done has been sufficient for me.
I also welded on a tab on which to mount a steering damper. Along with a fork brace and new steering, swingarm, and wheel bearings the bike has become a solid handler in bumpy high speed turns. Good fork seal, fluid levels and playing with spring pressure by adding shims also helped immensely in ridding the bike of that "swimmy" feeling. Hope this helps.
