This steering wheel centering procedure is just a bit different from the one described above. I find it easier to implement. It will quickly let you know if your steering gear is exactly on center. If the gear is not on center, you might have the problem that you are describing.
BTW, there are a number of other Chevelle steering papers posted at this sight: www.corvettefaq.com/A-car.asp
CENTERING THE A-CAR OR 2nd GENERATION F-CAR STEERING WHEEL
This procedure is valid for GM A and F-Cars (1970-82) with a steering system that has an intermediate steering shaft that connects the steering column to the steering gear. These cars are front steer (the steering linkage tie rods connect to steering arms that are in front of the road wheel centerline).
All of the Saginaw gears (variable ratio or not) are designed with a "high spot" right on center. The "high spot" is actually a slight interference designed into the gear set so that the on-center feel going down the road will be as precise as possible.
Here is a complete procedure for centering your steering wheel. It is very important to start with your steering gear exactly on center. Park your car in a place where you can work on it yet you will be able to drive it straight ahead a short distance to check how it tracks.
First rotate your steering wheel from full lock in one direction to full lock in the other and count the total number of revolutions of the steering wheel (X). Divide that number in half (X/2). Now go to full lock turn in either direction and come back exactly one half your number of steering wheel turns (X/2). Now your steering gear is exactly on center. Open the hood and take a crayon or piece of chalk and make a mark right on the top of the flexible steering shaft coupling. Now you have a mark that you can reference when your gear is right on center.
Now drive your car straight ahead a short distance in order to establish the position of the steering gear and the steering wheel with the road wheels pointing straight ahead. With the wheels in the straight ahead position, check the mark on the flexible coupling. (The mark should be right on top.) If the gear has moved off its center position you will need to adjust the tie rods so that the mark is again right on top.
If your steering wheel needs to be rotated clockwise in order to bring the steering gear on center, you will need to lengthen the left tie rod assembly and shorten the right tie rod. If the steering wheel needs to be rotated counterclockwise, the left needs to be shortened and the right one lengthened.
Loosen the tie rod adjuster clamps on both the left and right tie rods, then turn both tubes an equal number of turns in the SAME direction to bring the gear back on center. DO NOT turn the sleeves an unequal number of turns or you will change your front wheel toe setting. Please note that by turning the tubes in the same direction, the tie rods will lengthen on one side and shorten on the other.
With the wheels straight ahead and your steering gear on center, now check your steering wheel alignment. The 6 o'clock spoke measured right at the steering wheel rim should be within 1 inch of being exactly at the bottom. If the steering wheel is not in alignment, use a steering wheel puller to remove it and reindex the hub on the steering column shaft to center the wheel as closely as the serrations will allow. You can tweak the tie rods one last time to bring the steering wheel exactly on center.
Sometimes the flexible coupling can be removed from the steering gear and reindexed by one serration. If the intermediate steering shaft has a detachable lower flange (one that has a pinch bolt), it might also be possible to reindex that flange by one serration as well.
Note that these connections have pinch bolts that were designed to pass through a notch on the mating shaft. For the most part these connections were not designed to allow for rotational adjustment. They were designed to assemble in only one position but manufacturing tolerances may allow for rotating the mating flange by one extra serration and still allow the pinch bolt to assemble. HOWEVER, NEVER FORCE THE PINCH BOLT THROUGH THE NOTCH.