For the most part Model A frames are flat.

So let's assume that the car has no rake and sits level.

Orient the wheels to the straight ahead position.
Disconnect the draglink from the pitman arm.

Center the steering box by making note of the input shaft's turns.
Don't use the steering wheel to center the steering box, it may be off level horizontally speaking - and is one of the last things you adjust.

If the pitman arm is straight up - or down, depending on the steering box in use - with the steering box centered you should be able to connect the draglink with no problems.
If not, you'll have to re-orient the pitman arm to the vertical position. It can be off a bit, but vertical is what you should shoot for.

If the draglink cannot be installed due to incorrect length - either way, short or long - you'll need a new draglink.

The upper steering arm shown in the Speedway Motors link won't cure the problem unless your upper steering arm is considerably different. Perhaps custom made with the tie rod end (or Heim) connecting point off center.
It shouldn't be a problem since most upper steering arms are made with the tie rod end connection point in the center.

Do you have both spindle steering stop nuts installed?
And are they the same length?

Lastly, did you use the steering wheel to center the box?
As mentioned, if the wheel was simply plunked on in the horizontal position with no regard to having the wheels in the straight ahead position, then it's possible that could be the cause of the differing ratio's from left to right.