This is looking pretty sweet! Are you modeling in 3DS Max? If so, maybe you could give me some tips! ;) (I've been struggling with making a CGI quality version)
One thing I noticed that's off is the 'inside' area of the nacelles--you've missing the curved indentation at the front and back of the grills. See the two attached shots of the actual model in the Smithsonian. Also, the ribbing on the blocky things near the back of the nacelles should extend all the way to edges of the block, not inset as you've done. Oh, and the hull lines, if you must include them, should be MUCH less prominent.
Looking good!