I’m not an expert on frames, but these frames were engineered to flex. All of the components attached to the frame (crossmembers, spring perches, etc.), were riveted, thus avoiding stress cracking in the weld affected area. Welding hardens the metal, thus the reason you are seeing cracks. Welding up the cracks without heat treating afterwards, will create the same issue you are seeing now.
If it was my project, I would look for another frame. Even if you are planning to installing IFS, I wouldn’t want to start with those cracks. I see them come up here occasionally on the “Parts for Sale” forum.
Just my two cents!
