At the place I worked it was a 2004 ford e250 van with a 5.4L and it had 250K + miles flogged on it and it still ran great. I packed that van on more than one occassion where the rear bumper almost looked to be ass draggin. Im sure the van now has somewhere around 350 to 400 and still with original engine.
My 5.4 Expy(97) has 139k miles on it and hauled my cars no problems. Just be sure to keep up on your 3 main fluids: oil coolant and tranny! I have no reservations about the life of this engine, I personally think its more durable than the 300 I6!