I went thru this with a dash rebuilt by these guys . . ARRGHHH. . . they cover the dashes with a thicker material, making many joints difficult!
Use a dremel and a sanding wheel to trim both the dash where the surfaces interfere, this should only be done where it is completely out of site, and do the same to your ducts. Be careful, the plastic of the ducts is old and very brittle.
I would love to find some other companyt that can do BMW dashes, so if some one has experiences and can reference them please post.
It's to expensive to send one out for an experiment!