Hello, and welcome to Flyertalk, fairylights87
My first question is - why are you going to Mount Fuji?
Which location in the huge area around the mountain are you going to and what do you expect to do there?
Although Fuji San is very famous and well known I don't think of it as a natural tourist destination and certainly not in March, way before climbing season opens.
Also, do you have open jaw tickets (into Japan from Tokyo HND or NRT and out from KIX)?
If you have open jaw tickets then I don't see the point of JR passes. (You can get between Tokyo and Osaka pretty cheaply with
http://willerexpress.com/en/ if you want to save on hotel costs you might want to try the luxury overnight option, eg the cocoon seat here:
http://willerexpress.com/x/bus/dynam.../bus/index.php )
If you need to return to Tokyo to get a flight back home, then, sure a 7 day pass may very well suit you.