We have one guy looking to have a set of headers made up for his 320, but that hasn't happened yet, and frankly I doubt that it will. He talks a lot but we have seen little action from him. He is the only one that has even mentioned headers for the car except for those asking the question.
Custom may be the way to go, and if you decide to go that route, let us know, that may very well be the way to get yours paid for. If they can be made to fit the SLK as well, I'm sure that there would be a fair amount of interest at our site...
I don't know if the current manifolds on the 320's and the 32's are exactly the same, but if they are, there is (I think) a good market for a header system that has proven horsepower gains.