Get new cables - trying to extend them just adds another place for corrosion to interfere.
I would share Norm4015's concern about moving the battery inside. Even a sealed, valve regulated battery can gas under extreme conditions. And when you say "sealed", to you mean an AGM or Gel type valve-regulated battery? A standard "maintenance free" type of wet cell is not really sealed - it gasses under normal charging. It is definitely not safe for interior use.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL