That's funny they told you that, because they told me that it would not fit my 550 frame. My 76 550k is wider at the front than that seat allows (I think we are at about 9" wide at the front of the seat). That is not to say that you can't make it work, just that it won't be able to fit around the frame. You could build a bracket to make it work, I did not want to bother, and still looking for a seat pan to use and make my my own.
If you do go this route (I think others here have) post pics of how you mounted it.