From Stainlessheaders:
Quote:

Turn Your Custom Header Idea Into Reality With Our Headers Mock-Up Kit
Stainless Headers Mfg., Inc. offers the unique service of building custom headers to fit your application without having your car at our location. You can have a set of custom headers built to your exact header specifications using our PVC mock-up kit without your car ever leaving the garage.


I think their prices on header flanges were pretty reasonable for 3/8" thick.

Did you see their pricing page? Looks like $1500-2000+ easily.
http://www.stainlessheaders.com/pricing