I'm not sure about the coating part of your question, but i can't understand why TTI would claim that the headers won't work on a 318.
They may not be optimum for the port size difference, but claiming that they ''won't work'' on a 318 is hogwash.
There is lots of meat on the sealing surface of a 318 head to accommodate the bigger tubes and flange.
In fact, i do have a cheaper brand of header on my A-Body 318 (Flowtech, don't buy these, they are designed for a B-Body i found out later!) that is also designed for a 340/360 engine and had no issues with the sealing to the head issue, my issues were hitting the steering box and coupler, torsion bars, and oil filter. Cheap headers are a nightmare to say the least...........
I would say that a 318 sized port header wouldn't work on a 340 or 360 port size engine, not the other way around.
I would call TTI and i bet they say that it's a misprint.
