First off.. if you are actually seeing your arrows fishtailing then you have a very bad habit with your form because you are "peeking" at the shot. Let me ask you...are you shooting at an indoor range when you are seeing this "fishtailing"? If so I guarantee you that you are seeing an optical illusion because when your arrow travels down the shooting lane there will be a point where the lights hit off your fletching causing you to think they are fishtailing. When you shoot 20 or 30 yards do your arrows hit where you are aiming? Are they going into the target straight?. Only reason I'm asking you these questions is because I thought at one time my arrows were doing the same as you describe when essence they weren't. As far as your spine, I think the 340 spine would be good but you would need to cut your arrows down to between 29-30 inches and shoot 100 gr tips. Hope this helps.