.300 will be very overspined. I use a 120 gr tip and would have to go to a 34" arrow for a .300 arrow to work.

I've been playing around with OT2 and I still think you can use the .400 with your bow even at 70 lb. Your IBO will not be 320 fps with a 28" DL and therefore your performance will drop.As I move the slider down the spine goes up for your bow using the Carbon Tech Whitetails in .400 spine. If you wanted to make sure about this just add a wrap to increase spine. I'm guessing that your speed will not be over 280 fps with a 28" DL even with that arrow. Therefore, if your speed is slower than what I've used the spine will be stiffer than what is shown.