I'm gonna have to just knock on wood and hope its as strong as it needs to be.
I would have removed the stubs first then welded just the spider gear contact faces. That way you avoid heating them at all, and you can replace them if they break. You have now welded in the stubs into the diff. if you do break one, you will have to replace that whole diff unit, including having the new one welded.
Sweet project. I'd love to take it for a few laps in the dirt. looks fun as hell.