There are standalone python 2.7 tools that do what you want available in the Mobi forum here at Mobileread.
Grab the latest Mobi_Unpack_v045.zip from here:
It will allow you to split your Kindlegen output into mobi7 (original mobi) and mobi8 (KF8) pieces, as well as convert the KF8 piece to be an epub that should look exactly like what your Kindle Previewer shows you.
It may be easier to simply install the latest mobi_unpack_plugin. See the Calibre plugin thread for the latest version link.
To remove the extra source archive get kindlestrip_v133.py that understands mobi file produced by the latest Kindlegen: