The replacement of the flappers is an aggrivating job but not too bad if you dont have big hands. I would recommend you go ahead and replace the rubber gaskets under the flappers as well while you are there.   As Dave pointed out, an assembly and shop manual will be a big help in seeing how every thing works.   Removing the plastic boot that covers of the door latch on the inside will be necessary if your 78 has one (not sure when they started using those).   The most difficult part of the replacement is the removal of the nut closest to the rear edge of the door on the old and replacement of the same nut on the new handle.  Just try to be patient and use the smallest wrench you can to get in there to "break" the nut loose, you may have to grind excess material from an old junk wrench to get in there. Disconnecting the rod running from the door lock button to the latch will also give more access to the inside of the door.  Duct tape the rod to something to keep it out of your way till you are ready for it again.  If you have any long  surgical forceps they are really good for removing and replacing the pesky metal clips and reattaching them as well as maneuvering the flapper rod and pin when you are reattaching it. Leave the threaded rod attached to the flapper when you remove the old one.  It may help if you can hold the flapper down as you withdraw the flapper and attached rod from the door and do the same when inserting the new one.  A little masking tape around the area where you will be working is cheap insurance against scratches while you are fiddling with the flappers. The little metal retaining clips that you are sure to drop while removing and installing will end up in the bottom of the door.  There is a removal access plate at the bottom of the door that can be unbolted to let you retrieve them if a long magnet tool doesnt work. Once you can see inside of the door, the job is fairly straight forward but yes, it will be aggrivating and one where blue language is appropriate.  Sorry for the long post guys, but you know I can't say "Hello" in 100 words or less

  Removing the plastic boot that covers of the door latch on the inside will be necessary if your 78 has one (not sure when they started using those).   

They had them in '73 (mine does) And yes that nut in the back ...