Starting at 60 @ 28", you will be close to 55-56 lb at 26.5" draw. I went to the Mowhawk site and it doesn't say, but his bows look to be cut pretty close to center to about 1/8" out. I couldn't tell about the string either, but I suspect it is a "hard" string. Your correct spine will depend on arrow length, point weight and you, but I would start with a 55-60 for a 28" BOP arrow.
A set of "test" arrows is a great help in finding the right spine. One good option would be to get a half doz or so shafts that spine from about 52 up to 62 lb and make up your first batch of arrows with these. Play with arrow length and point weight to find what you like and then order some matched shafts. If you have a hard time finding the shafts, let me know and I will probably have some that will work.
Enjoy the journey!