After replacing a feathing shaft...how tight should I be tightening the two nuts on either side of the shaft (in the blade grips)? The tighter I make them the less side to side play the blade grips have, not sure what to do?

you want those all the way tight so the blades don't come off in flight. Also the side to side motion on the blade grips should be basically 0. You still want the blade grips to roll quite easily though.