Cleaned up and installed the Maxim Works header. It really was just a thin layer of surface scale, because a few minutes with a wire wheel knocked it off without much trouble.
I also sourced a couple of esoteric pieces for the exhaust. A flange with bolt holes wide enough to match the header I couldn't find anywhere but ebay, and to find the round and 'flat-in-cross section' gasket that fit in the groove of the MW header's flange involved taking a pair of calipers to NAPA (lol)
Fortunately, if anyone can help find something like that, it's NAPA. Good luck at most other Auto Parts stores, where the first question is "what's the make and model," and "it's not a stock application" is met with "the computer needs to know...." Useless. The guys at NAPA actually broke out The Book with all of their exhaust parts and pieces. Super helpful folks, and now I have the actual part number in case it ever needs to be replaced.