You can purchase a couple of different pre-made short shifters from
Rodney Dickman.
If you have access to a welder, it's easy to shorten the original shaft & re-attach it to the base. Not to mention that cutting off the old threaded end & welding on a bolt with more common diameter & thread pitch will make fitting aftermarket knobs easier.
Whomever designed the shifter may have had penis envy... the threaded end of the shaft is larger in diameter than just about everything out there, including other GM shifter knobs.