You probably know more about this than I do, but the reviews on Amazon were not exciting, and they mentioned unless you purchased it from the right place (not Amazon), you only receive a five-year warranty, not the lifetime guarantee Amazon states, and several people said aftermarket manifolds failed within a few years. I swear I once saw a hairline crack in my manifold by the upper oxygen sensor, but I took it to a welder and he did not see it, and he said it was square.
I tried JB Weld, which simply disappeared, but is only rated for 550 degrees. These products are supposed to be good for 2,000 - 2,500:
POR-15 Fire Seal 2000 FS2 - Free Shipping on Orders Over $99 at Summit Racing
Amazon.com: Blue Magic 18003 QuikSteel High Temperature Metal Repair Blister Card - 3 oz.: Automotive
Amazon.com: Blue Magic 18003TRI ThermoSteel High-Temp Metal Repair - 3 oz.: Automotive
I would personally prefer an OEM header welded by someone that knew what they were doing to an aftermarket one, but I welcome the opinions of more knowledgeable members.