I believe southbend called for .010” end play when going to the larger input shaft. Idk if it’s for the extra load and heat expansion they figure you’re probably putting through it with extra hp, or if it has something to do with the way the clutch and shaft load the bearing. And if you never figured out your going into gear, if it was just low gear, could be the synchronizer for low and first being flipped around. There’s a recess machined into it on the low side for it to actually slip into the gear cavity and grab low, otherwise it feels like it takes but just spits it right out. Ask me how I know