Yep you're missing the factory service manual. The 1972 book available here (pirate copy arrrrrr!) will get you by until you can get hold of the correct '68 book.
I actually have the factory service repair manual for it, and it makes no mention of removing the box in an AC-equipped dart. What it shows for a 68' Dart is the non-AC version, and the pictures of the heater motor mount are different than mine.