It sounds like your shift rod washers are toast. At least, I would replace these first and go from there. An easy and cheap fix. After all these years, they are most certainly shot.
Check out this thread for some DIY information.
M-100 Message Board - Horn "short" in column change cars.
Note that the tutorial is for a different chassis, right wheel drive and your car does not have the collapsible bowl behind the steering wheel. But the principle is the same. Also do a search. This topic has been explored previously.