Well after the kitchen door was replaced I was very happy with the way the air leaks were gone, my next move was to replace the den door, the door was split (dutch) style and leaked air, I had masking tape on the inside to stop the drafts.
The den was added onto the house and not from 1792. It appeared that the construction was set to the normal 80″ height, however the owner decided to install a 78″ door so they added a 2×4 on the header, I was happy to just remove the 2×4 and some drywall to fit the replacement pre-hung door, another surprise I found after removing the old door was the flooring consisted of 3/4″ plywood and 3/4″ boards glued to the plywood. All in all much easier than the 1792 kitchen door replacement. All that is left now is to remove the brick mold and install 5/4″ trim.
So now I have 2 new doors installed on the back of the house, hopefully my heating and cooling savings will add up.