It appears I cannot get there from here.
I found a
discussion on a Chevelle forum that spelled it out:
"The shift bowl is a press fit into the shift tube which extends down inside the steering column. The shift tube is rather delicate since it is designed in two pieces and is supposed to telescope onto itself in a collision.
I don't know of any easy way to get the bowl off of the shift tube without special GM tools."
It seems the manual shift column I have must be from a 67 or 68 and will not be easy to adapt to my 70, nor maybe smart. For now, I'll just remove the shift lever and indicator from my original column and accept it for now. I am pretty confident I can keep the back drive and original switches working with the aftermarket shifter, and obviously I can keep it with a stock shifter if I go that way.
I'll keep my eye open for a used manual column correct for 70 and if it gets frustrating, I can always spend a lot for a remanufactured unit.