You will be shooting 55-56 lb at 28", and I will assume a 29" BOP arrow. According to the website, the Kanati is cut to center, so add 10 lb, 5 for the inch of arrow over 28, plus another 5 for the 160 gr point and hard string. I come up with a 75-79 lb spine. You might do fine with a 70-74, but it is better to be a bit overspined than underspined. If you are making your own arrows, you can start a bit long and trim them down until you get perfect flight.
Go with a mid weight wood like fir, pine or ramin and you will have no problem getting enuf arrow weight. A 75-79 fir raw shaft will go 450-500 grains or more.