The U joint, that's why longtails don't have reverse!
everybody knows thatSeriously, make certain you have a flat spot ground into your shaft for the u-joint bolts to grab and not mar the OD of the shaft( de-burred super smooth so you don't damage the seals when assembling or changing shafts).
Safety wire the u-joint bolts. You can get a wire twister and some stainless wire cheap at Harbor Freight. http://www.harborfreight.com/9-inch-saf ... 45341.html
You'll also want to peen the key so it stays put. You can tack weld it if you don't get carried away and don't mind grinding it later and again deburring.