George
This article at Nikonians seems to answer your question directly:
Manfrotto 393 Gimbal head* (1) - Nikonians Product Review
As the article's illustrations show, you can invert the top section to achieve the reverse mounting you were concerned about, assuming you have the appropriate sliding plate that came with the head. And with a weight capability of 20#, I would assume this head can easily accommodate your new lens, particularly since the writer is using it for a 600mm f4 lens. Good luck.
PS This was the first reference article on the page when I "Googled" your head.