Here are all the packages specific to the Fluxbox edition: http://packages.linuxmint.com/list.php? ... ra-Fluxbox
I'm not sure if they are present in the main edition's repos but you can check. Otherwise, just install the packages manually. This is not supported so it might break something in your Gnome installation. I'm sure Kendall has taken measures to make this as least likely as possible but you should be aware. For starters install only the essentials... fluxbox, tint2 and mint-fm2. These should be safe. Then see where you want to go from there. You can also download the default settings package and use the config files from there to get a more Minty feel.