Be careful what you weld up.
Remember that certain places SHOULDN'T be welded.
The corners of gussets, for example.
I was considering doing the same thing, but somebody mentioned that new bracing and gusseting would likely crack and bend our old, fatigued, soft-steeled frames and welds.
Make one area too stiff, and other parts have to flex even more to compensate.
After careful consideration, I realised that only 1 stiffening mod to teh fram could be made without causing too much stress on various joints and frame member...
You'd have to run directly from headstock to where the swingarm bolts up.
Would have to go around the engine. Would make a kickstart unusable. Would be a pain in the ass. But it would really, really stiffen things up, without overstressing any old frame part.
Could make them bolt-on, for removal.
BMW did this on their 1976 r90s superbike racer.
I'm not going to bother. Too much work.