Agree with the above. You rarely see a manifold completely red, you will occasionally see a seriously high boost build getting close to white near a restriction but I've never had that even with some pretty big builds.
Mild steel is dull red at 500deg and cherry red at 800deg. However be aware that manifold flange warp could actually be caused by a delta temp. Ie, the head will be a lot cooler than the log section of the manifold so there will be a temperature gradient through the thickness of the flange. Knowing how they normally warp would tell you if this was the case or not. In some cases you could actually make the issue worse by heating the whole thing uniformly as thats not how it is in use.
Rob,