That happens when you lower the front, even a little bit. It will change more when you add the extra caster to it. I had threaded rod made with left and right hand threads, then added tie-rod end to it. This went in place of the fixed draglink so I could dial in the steering box dead on straight ahead. You will have to come up with something similar to correct yours. I bet it turns more oneway then the other. It makes a big difference in how they drive, and if your gear box has variable ratio steering it will be worse when not on center.
Once you get the caster shims in, hookup a couple ratchet straps to the axle and bumper and pull the axle forward before tightening the u-bolts, this will insure the axle is fairly straight as there is some slope in the locating holes. As you tilt the axle with either lowering or caster, it moves the steering arm closer to the gear box, this is why the wheel is off center.
The attached drawing from the '37 Engineering manual shows the arc of the axle verse the steering.
