I took a look at your epub (thanks for your work!) and your internal style is quite different from the SE house style. If you were to convert this to an SE project, you would still follow the
step by step guide, only instead of splitting a Project Gutenberg text, you would copy over the HTML body of each section (apart from some things like your production notes and the copyright page). Judging from experience, converting your epub to SE style would take about as much work as converting a Project Gutenberg text. Alex (the head editor) will be able to speak as to whether this would make for a good first project.