I can't comment on the historical society aspect of your question, but you
will need to file a 501c(3) with the federal govt to obtain nonprofit status.
I would strongly advise you to enlist the aid of an attorney experienced with
nonprofits.  You want to be sure to do everything properly right from the
beginning.
  There is a Center for Nonprofit Assistance (or something like that) in the
Phila area.  I think it is a national organization.  I don't have their name &
# at my fingertips at the moment, but can get it for you later today if you
are interested.
  Barbara