Here are some more exploded views of the upper column from the Master Parts Catalog. Is it the part no labeled 386789?
The GM part number started out as 5673012 then was superseded by 386789. Tried to find one online with no success. One may turn up for sale on eBay etc with continued searching. It's not a maintenance part that wears out, so it wasn't stocked very much.
These spring nos were also used in other Buicks, some Chevies and Olds as well. Most usage was in the '64-68 period only in non-tilt columns. The spring could turn up where someone is parting out one of these cars. There's always the salvage yards too.
Some Chevy catalogs show the free length as 1-3/32" and also as 1.01". ID is given as 0.77". Could not find anything on wire diameter, number of turns, or spring force rate.
The stationary contact plate inside the steering wheel is grounded through the steering wheel core, the steering shaft, the shaft bearings, then to the car frame. Bearings aren't a real good way to carry current; there may be an extra long wire in there somewhere to ground the steering shaft.
Attached is the shop manual description.