From purely bureaucratic point of view, the best time to change name is after marriage, when applying for GC through AOS or when applying for naturalization.
It's a bit less desirable in my opinion during I-751 stage. However, since you're already in it and have mix up of last names in various documents, I'd use new last name on the forms. I'd also include copy of marriage certificate just in case as well as short cover letter explaining you've changed the name.
If everything goes smooth, you'll receive GC with the new name.