No one has posted yet so I will at least suggest that if a Bentley manual is available that covers the '78, that is the one I would look for. However, you cannot have too many manuals so supplementing that with a Haynes or factory manual would not hurt.
My car is a bitsa that spreads the years from '71 to '75 (or so). I use a Bentley manual, Haynes manual, and the Heritage manual on CD (old XP laptop required).