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Well that's promising. The chainstays are very thin on 653 frames though, at 0.6mm. What's the thinnest tubing you've achieved this on?
Ndeipi-
I’ve welded 531 at the thinnest and 531 isn’t a good steel to weld, even so I wouldn’t hesitate thinner. You can weld razor blades, the thin aspect isn’t the hard part. It’s getting the metal clean enough and the inside of that tube is likely to be all scabby.
I’ve welded up my very old vespa which needed lots and lots of welding on the thinnest crappiest steel panels. It is doable it can just be a pain if the metal is poor quality or dirty.
The good thing here is that it’s a tight crack, you may be able to run a weld over it and close it up without any filler and it will be fine. So long as it’s clean... from what I recall 653 is a good grade for welding too.
Short of it though is that it is very possible.
Dogtemple
You can run a weld in to that. It would be unnoticeable when done. I’ve welded up a couple frames with big cracks and still holding up years later.