To the best of my knowledge the splines never changed from when they started using them (in the 30’s?) until at least 1984, but there are at least two differences between the 55-68 & 69-84 attachments. The outer diameter of the later column is larger than the early models. In order to make the early wheel look correct, there is an adapter kit that provides a ring to attach to the back of the wheel, which matches up the size of the wheel to the larger column.
http://www.classicchevy.com/chevy-st...1955-1968.html
The bigger problem is the depth of the turn signal mechanism and locking/hold down plate within the newer column (essentially how high up the shaft sticks out of the column. I bought the adapter kit for mine, and attempted to install one of the smaller retro 59 Impala wheels. My column is a 69-84 aftermarket model from Butch’s Rod Shop (long since out of business). Some of the 69-84 columns are like mine having the turn signal mechanism and upper locking plate set too deep in the housing, while others have the switch and plate assemblies mounted nearer the top of the steering shaft.
Even though the splines matched up, it was impossible to install the wheel since the plastic back of the wheel ran into the column housing before the hub could seat on the splines, and I could barely get the nut to engage the first threads on the shaft. With or without the adapter ring, the wheel hit the column and when I tried to tighten the steering shaft nut, the wheel was clamped to the housing and could not be turned. If this is the problem you’re facing, the only option I know of is to replace the column with a something different. My budget was stretched to the limit so I gave up and sold the wheel instead. Someone else may have a solution to this problem but I didn’t come up with one.