What I do is:
Jack the front of the car up and put it on jack stands so the front tires are just off the ground.
Start the car up and loosen the bolts that hold the block on top of the steering box.
Tap the block slightly to the right or left (about 1/16 of an inch at a time).
Snug down the bolts and turn the steering wheel all of the way to the left and right and then turn back to center the wheels.
Then let go of the steering wheel and if the steering moves to one side or the other, adjust again until it stays centered.
I have had good success by doing it this way on several cars and trucks over the years, it just takes a bit of fiddling around to get it right.
I would start by moving the block in the opposite direction that the wheels self turn for starters.
Good luck, and let us know how it worked out for you...