An existing shaft cannot change is length. There are 3 shaft sections that all function together and cannot be shorten or lengthened without impacting function of the spotlight. They also cannot be switched out from the headpost assembly (that is the part that connects the shaft to the head of the light). The shaft needs to be staked into the head post assembly using a special machine. I am not an engineer, so it may be possible to do it without the machine, but I have absolutely no idea how and have never heard of anyone doing it.
The best way to switch the shaft would be to change to a different shaft assembly. Available shaft lengths are on the chart below. For the 1970 Dodge Polara our application engineers recommend a GM length shaft. Any change away from a GM shaft we do not support as this length was chosen for proper function and safety. You would need to decide which shaft you think would make the most sense for you personal preference. When you think you have landed on the shaft length you like, I can direct you to the correct part number for the shaft assembly, but I cannot recommend a shaft other than GM length.
Exposed shaft – the amount of tubing visible between the head housing and handle housing when the spotlight is assembled.
Center to center – the distance between where the gears mesh in the handle and where the gears mesh in the head when the spotlight is assembled.
I should probably add that is if the light was installed in the correct location with a 204 installation kit on driver side or 204RH kit for passenger side. If different parts or mounting location were used that could cause issues with shaft length.