Random Thought: Heart of Darkness was written at the turn of the century (19th Going into 20th) so, I think Apocalypse Now was written just as much from Coppola's own writing as it was based upon the novella.
Meh... It's dicey. Conrad wrote HOD as a response to Imperialism in Africa. Coppola concepted Apocalypse Now for Vietnam, so it was done with the same intention