I have a 52@27 MOAB, and shoot 45-49 hexpines (11/32) out of it, cut at 28". That being said, I also shoot 5/16" 50-55 Laminated birch cut at 27 BOP out of it, both with 125 heads. My latest batch of arrows are 5/16 hexpines, 28" long with 200 gr on the tip, spined at 50-55. All of these combos fly like darts, weight from 565 gr for the first set, and the other two sets weight around 620 gr. All shoot fine, MOABs are very forgiving as far as arrow spine.
I'm not 100% sure, but seems to me Jim cuts his risers 1/16" short of center shot. These reults may vary slightly depending on your draw length, and the length of your bow.
For me, I've tried stiffer shafts, and they flew stiff. Good luck.