450 street frames had flat metal gussets either side of the back bone spine and ran down towards the swing arm pivot forward of the battery tray.
Supposed to help with lateral flex. It does little to stop this but does make fitting a descent carb and manifild very hard.
The 350 didn't have this (I think) and the 250 definatly didn't.
The other weak area is the swing arm itself and the area at the rear of the engine.
Be smooth and it doesn't matter.
A proper 500cc would be a different thing.
If you need a proper race frame, there are people around that can supply them.
I played silly buggers with a 250 frame and some chrome molly tubeing years ago. and a cut down SS swing arm as I was sure I could go faster if the frame didn't flex..... I didn't.
You can see in this picture of a replica Spaggiari frame that there is a brace on the rear of the main tube welded all the way down to the subframe portion and curved down to the engine mount.
That is the way I've seen most race frames braced. Classic ducati also used to sell a kit.
I may know of a set of braces already made up if you are interested, my friend may want to sell them.