5.29s seen to be the hardest to install, the tips of the ring gear teeth stick out farther. Either grind the notches in the housing a little deeper (there are two pieces of steel there, often the two don't line up perfectly and there is a little step - grind that flat) or try rocking the pinion flange back and forth as you work the 3rd into place, that's often all it takes to get the gears to fit into place.