Lowering it via a Car or Truck and a stout Rope tied to it's Bumper of Trailer Hitch, of course is excellent, if you can figure out a way to securely mount a Pulley maybe, for the Rope to pass through since there is no direct shot at it. Finding appropriate old time Pulleys now a days though is not always easy unless one already has them on hand.
If no Pulley is to be found to serve, then the Rope could be merely provided with some Boards or something tacked on to 'corners'so it does not chafe against finished surfaces, to slide against for going around corners or doorjambs and so on...and, be passed winding throuh the House that way.
Most older Wooden Stairs to-a-Basement, if built right to begin with, would handle the weight just fine, as long as the weight was being distributed by the Safe being on it's back on skids/2x4s or similar.
Is there enough room to lay the Safe on it's back or side at the top of the Stairs? So it is pointing downstairs?
But, do inspect the Stairs from their underside, the Stair Jacks or long notched Boards which hold the Treads, so see what they look like...and, if they are 'iffy', ( cracks, knots, spike knots, whatever ) then consider to shore them up using 2x4s from below running vertically, and you can just 'tack' those in so to speak, cut their ends to maatch the angle of the underside of the Jacks, and, Nail them leaving the Nail Head sticking out a little for easier removal later, just so they stay put for the procedure. The 2x4s would have to run from the floor to the underside of the Jacks, straight 'up'.
Long as you can somehow get a stout Rope secured to something, somewhere, even if passing it through a doorwar or through a Room or Hall to tie it to a Car or Truck which shall remain stationary and only serve as an Anchor, the end of the Rope or a Loop of it, could be used to attach a Come-Along or adeaute rated hand Held ratcheting Cable Winch, or a Block and Tackle, for lowering the Safe down the Stairs safely.
Taking the Door off the Safe would save lighten it by a few hundred pounds, as johngross mentions, so, well worth doing..!