Replacing original 1950s exterior door with new prehung: threshold question
Most if not all doors available today are sold at a 6' 8" standard height. There are several widths to choose from. The new pre-hung replacement door should have a slanted sill attached to it's jambs. So, when you are ready to remove the old door: remove old door from entry (remove the pins or unscrew the hinges),  remove exterior trim from top and sides. (This will expose the edge of the doors jamb). The easiest way to remove the jamb is with a saber-saw and cut the fasteners securing the side and head pieces from wall (check for electric runs and water lines). The jamb will tilt out in one piece if cut all the nails. You now have a hole in your wall, but technically is labeled a "rough opening". Take measurements side to side and from the bottom of the header to the sub-floor and decide on the door size.