Not really the same problem as yours but similar, my '37 always went down the road with the steering wheel off center 1/8 turn, so mine would turn more one way then the other. Everything was rebuilt or new. I never could solve this, figured the newer axle someone installed was causing it. I had new spring and bushings, steering components, gear box fully rebuilt , and steering wheel right ( '37s use a keyway on the steering wheel). To solve it, I had an adjustable draglink made using two tie-rod ends. It was easy to center the wheel and the truck drove much easier since the gear now helped hold the center. When I changed the caster angle from 2 to 4 degrees, the wheel went off center again, so a simple adjustment brought it back into alignment.
It really doesn't take much change in the axle position to change the steering. Double check that the caster shims are facing the right way, thick ends towards the rear of the truck. It could be as simple as loosening the u-bolts and pulling the axle forward is enough to correct it, or pushing it back, depending on which way it needs to go. The alignment pins are not super precise.
