Geoffrey, if you are gonna pull the steering wheel and replace that shift tube selector arm, look at the Technical Resource Library and it will tell you how to do it. In fact, here is the link.
Here is what my worn shift tube selector arm looked like. You can see the wear in the ring and in the inset. This is one of the major causes, after some 50+ years, of sloppyness in the column shifter. You can buy them from any of the major Tbird parts houses. The Bird Nest has them. Part #7203A for $29.95.