Steering shaft U-Joint question?

Non splined? I would put a tapered pin in it, small end at least 1/4” using a tapered reamer. That way it will not come out. I would not hesitate to put a few tack welds on it using a stainless rod cause it will weld dissimilar metals and not crack out. Make sure you peen the welds right after welding to stretch the weld as it cools and shrinks. Stainless rods are easy to weld with.