RE: 4 inch or 5 inch fletching
Using the weight calculator at Jackson' s to figure different arrangements for your CX 300' s:
4" feathers would give an arrow weight of 366 grains and 12.3% FOC. 5" feathers give 369 grains and 11.9%. 4" vanes run out at 384 grains and 9.8%.
Just based on my personal prejudice as a long time plastic vane hater, I' d recommend 4" feathers with a healthy helical. Especially since you' re shooting a dropaway rest. IF they don' t stabilize the arrow good enough for you, and I' m almost certain they would, you could always go to 5" .
I definitely would NOT go to 5" vanes on a carbon unless you bump up at least to 125 grains on tip weight to keep some kind of decent FOC. Besides, they don' t give a broadhead arrow any better stabilization than a 4" feather does, IMO.