As far as I can tell, the problem exists entirely within Final Cut Pro. Because even a straight-up import of xml causes it to freeze at 67 or 68%. Which is rude and ignorant of FCP.
And I changed the last value to "1" which should be the 1.66:1 letterbox. I did a search and replace. Very proud of myself, I was.
To no avail though. The import still froze at 68%.
So I quit. And I'm doing the changes manually. It's not as difficult as I thought it would be.