I really dont see how a 400 spine arrow is underspined for your set up. It should be perfect at 29 inches with a head of 100 grains. 125 should be fine too. If you were really concerned, and using a 125 head, cut the arrow to 28 inches. I shoot a 28.5 allegiance at 80 lbs and shoot a 28 inch 340 spined arrow with a 100 grain tip very well. I dont see that a 340 spined arrow is needed out of a 60 lb set up at 28.5. You would be over spined.