Pull the whole column. Remove the top two collars, wiring, etc. At this point you should be able to remove the steering shaft tube from the bottom. Bottom collar is held in place onto the shift tube by an allen key set screw that connects it to the shift tube. Once disconnected the shift tube comes out same way as steering shaft tube. You will have an extra wire in your harness from the PRND2L selector light that is no longer needed, and can be removed by collapsing the tangs on the crimped connector inside the harness. I use a small jewelers screw driver or paper clip.
Do the aftermarket columns come with the return spring for the bottom collar? The factory floor shift collars from 71 and forward have a return spring that hooks into a small hole in the outer tube and the other end of the spring has a loop that connects it to the bottom collar. There's also a small rubber bumper that attaches to the bottom collar. There's also a spacer that presses into the outer tube that helps support the bottom collar.