when you seam/stitch weld, you leave some play room for chassis flex that is necessary. Think of it this way... if you fully welded every frame component on a car, and took it to a track, you'd have cracks all over the place despite the car being very stiff.
jon's right about seam welding, and I should've asked the shop to stitch weld it, but as he said, it won't cause problems for me because my car won't be track only balls out racing every weekend. It won't see that much stress