The last coulpe of times driving my 450slc I've notice there is a looseness in the wheel. When the car is stationary and I grab the wheel at 12 and 6 o'clock position and pull it towards me there is about an 1/8" give and take. If I turn the wheel slighly to the left and right I hear and feel a slight knock. I don't think its in the coupling as I don't have hardly any play in the wheel. I feels like its the column itself. I haven't taken the steering wheel off yet, but I'm thinking maybe the shaft nut is loose. Anyone know what else it could be?

Could be the wheel fastening nut, but, if not, check for a worn knuckle, then to Pitman arm, tie rods. Was the in/out play present in the wheel before you heard the sight clunk? I kind of doubt the clunk is due to this endplay, unless the splines on the shaft are loose.