What are some important quotes from chapters 6-8 of Into the Wild?
I would personally say that there are two important quotes in these chapters, but of course, you have asked a very subjective question, so I would recommend that you read these chapters yourselves and try and relate their contents to the overall theme of the book and pick out any important quotes.
Firstly, I would argue that the letter that Ronald Franz writes to the author where he comments on his impressions of Chris McCandless is significant (page 49 in my edition):
Please do not make these facts available to anybody till I know more about his death for he was not just the common wayfarer. Please believe me.
I believe this quote is important because it indicates the massive impact that Chris McCandless had on those around him, especially Ronald Franz. It also makes it harder for us to just casually dismiss Chris and his exploits as being a stupid stunt. Franz's belief that there was something "special" about Chris and his philosophy and approach to life necessitates a closer examination of his motives and beliefs.
Secondly, and very interestingly, the comments made by others about Chris and his lack of common sense are worthy of attention, as they suggest a character with more brains than common sense. Note what the text tells us in Chapter Seven (pages 63-64 in my edition):
Nor was McCandless endowed with a surfeit of common sense. Many who knew him have commented, unbidden, that he seemed to have great difficulty seeing the trees, as it were, for the forest.
Although he was said to not be a "space cadet," many agree that there were "gaps in his thinking." As we struggle with the author to develop an analysis of this confusing and troubling figure, this is another piece of vital information that points to the many different aspects of Chris's character: part visionary, part Romantic, part intelligent, cynical, world-weary youngster in search of a purer way of living.