I finish all (well, most) of my books in Word, first 2000, more recently 2007. I find that entrusting a Word doc (or *.docx) to the Smashwords Meatgrinder or any other auto-conversion is an invitation to messy html and very often an unsatisfactory book. What I would do is run that *.doc file through Word2CleanHtml dot com online, open the resulting html file in Sigil, and tweak it there as needed. Takes some learning, but easiest in the long term. You can get the latest version at
https://github.com/Sigil-Ebook/Sigil/releases.
Toxaris also has a plug-in for recent versions of Word (I think it's 2003 onward). Do a search on the Sigil forum for that. Really, Sigil is the best thing that ever happened to technically challenged self-publishers.