Did you look at this site: Airstream, Inc :: 1970's
? Some of the catalog items are listed in different years.. I am pretty sure the Excella 500's were a 31' version at slightly lower trim level than Sovereign's. I thought they appeared first as '73 models, but someone else can weigh in..
Old roof vents are semi-worthless (more noise, ,less air moving..), but they are generally wired for 12V
power and fit 14" square, which should accommodate a Fantastic fan with some effort. We had dealer install 2 in our '88, replacing one motorized fan vent and 1 basic crank-up vent lid.. (WELL worth doing!) If rear bed fits in sideways as shown on Sovereign
floorplans, then there may not be either width or length to upgrade to King bed.. Only way to tell for sure is to get into trailer with tape measure and see what space there is.. Be careful also about either wheel wells, plumbing work or external storage compartments designed to sit under bed, as those may limit your options as well.