The more fletch, the more quickly it stabilizes (and slows down) the arrow. Of course, a well tuned arrow that leaves the bow with less flexing is better than compensating with a lot of fletch, thoughthe latter does work.
Why are you shooting 105# cedars out of a 60# bow? Are you sure they are 105#. Aside from Sweetland forgewoods, I've never seen standard size cedars make that weight.
Even with a fast flite string, and cut to or past center shelf, something in the 80-85# range should work. Could also build out the arrow plate to accommodate lighter spined shafts.