New engine, glowing header tube

[Gregsenne]

My engine is brand new, and I tried breaking it in, and the #8 cylinder header tube was glowing. I have a couple ideas of what it might be...
Its a 350, 40 over, carbeurated.
Anyways, can I break the engine in, at 2000 RPMs, with the header tube glowing cherry red? I want to get the engine broken in, and then I'll mess with why it's glowing. But I want to make sure it will be okay to leave the truck running 2k rpms for 20 minutes with that tube glowing.
Will I have to worry about damage to the headers? More importantly, will I have to worry about cracking the head around the exhaust port?

Thanks.
Greg