On a floor shift column, the lower third section of the column (some refer to as collar) is actually held in place with a set screw to the shift tube. So, you can shorten the tube where it sticks out on the engine side, but it needs to remain. If you remove the tube, there's nothing for that section to attach to. On a column shift, that section contains the lever, boss, spring, etc.
The middle section with the key cylinder is screwed to the outer tube with long sheet metal screws. You can see the shiney heads in the photo below.
In the photo below, the shift arm has been cut off the bottom of the shift tube, but you could cut the tube back to the end of the outer jacket. The plastic bushing should remain to support the end of the shift tube.
Backup up a bit, below that third section there is a retainer pressed down into the outer tube. A wave washer rests on top of that with grease. That acts as a pivot point and STOP for the lower section/shift tube assembly to keep it from dropping down too far (or separating down and away from the middle section). Clear as mud?