Obeamweldor,
It's my fault. On your first posting you wrote "fin fan HEADER box" and I mislooked the word "Header". So, I thought you were talking about the pressure inside the steel frame crossed by the air draft. Here, the pressure very seldom is higher than, say, 20 inches of water column. Such a small pressure is negligible for the tubes and headers, so forget about it.
Now, let's go back to the headers you were actually talking about. The header consists of several flat faces, with one of them having the tubes attached to it. ASME VIII doesn't cover such kind of flat face. However, a shell and tube heat exchanger has a tubesheet that is exactly a flat plate with a bundle of tubes attached. Shell and tube heat exchangers are designed to TEMA standard (Tubular Exchangers Manufacturers' Association). Why don't you use TEMA to calculate your header box?
Giovanni S. Crisi