Darcy,
Did you take your roll pin and shift lever out of your column?
I'm not sure what the setup looks like on your early model, but the following photos show how you can replace the spring on a '74 Duster column:
First, the spring is right at 1" long x 7/16" diameter. The spring recess in the housing is about 1/2" diameter, so there is plenty of room to slip in the new spring.
Grip the spring tightly with tweezers (needle nose pliers were just too thick to slip between the coils) and ease it through the lever hole in the housing.
Rotate slightly to settle the spring into its recess in the housing.
Slip the shift lever through its hole, pushing back on the handle so the tip clears the spring and doesn't hang up or dislodge the spring.
Drive the roll pin back in and you are done.
If yours is different than this, then.... oh, well.
I would think you could find a spring somewhere like LOWE'S, Home Depot, or a well-stocked hardware store.
Good luck,
Jerry