Your problem may be more involved than just the backdrive system. Your ignition lock cylinder in the column head connects to a small plastic gear. That gear engages a small die cast rack that pushes and pulls on a rod that extends down under the dash to the ignition switch mounted on the steering column.
You could drop the steering column and remove the ignition switch from the column. Now you could determine if the key will rotate all the way to LOCK and OFF-LOCK. If it does, the problem was the switch itself. If it still binds, the problem is in the lock cylinder or in the plastic gear and rack.
The plastic gear or the rack could be broken. You didn't mention if you had a standard (non-adjustable) steering column or a tilt. Both columns have plastic gears and racks but they are different from each other.
I have written several papers that walk you through the disassembly and repair of the Saginaw steering columns. The papers can be found at www.corvettefaq.com/A-car.asp
The steering column head and lock cylinders on the GM A-car and F-car steering columns are nearly identical. So the A-car papers (either standard or tilt) should be most helpful. If you need a plastic gear, rack, ignition switch, or lock cylinder, all of these parts are still available from either GM dealers or from the automotive aftermarket.
The 1969 function locking, enerby absorbing steering columns were specifically designed to deter car thieves. By definition, this makes them more difficult to service. So it may take a bit more time and effort to disassemble one of these steering columns, but it is very much within the capabilities of a mechanically inclined owner. If you require additional assistance, help is only an eMail or a posting away.
[ 06-03-2004, 06:09 AM: Message edited by: JIML82 ]