I have had a stack of YZ250/465 G and H frames, all rust and then crack in the same area. Also check out the left side rear subframe strut where the chain has been sawing away for the last 30 years. I suspect IT's of the same vintage would suffer the same ailment as well.
The worst part of all, is that when you remove that gusset, you often find the frame down tube is rotten as well. This is a much bigger issue than the gusset itself.
Let me see if I have some pics of that area, I have some nice x rays of the frame gusset area around here too.