I'm not a bob or norm wannabe but there may be another option without looking like you built for midgets. Just cut the inch or two you need out of the top header with a reciprocating and a circular saw. If you have 2by6 headers a couple inches won't matter and if you have a flat 2by4 for a header then cut it out and toe nail it back in between the drywall. Door trim will cover any new drywall screws. You just want the door bottom to be just above any carpet or tile so it doesn't rub on it. Cut off or shim up the bottom of the door jambs so the door bottom fit as hung. Match up the frame and opening and measure each one before cutting. Good luck.