On the rearmost bottom, there should be a landside that is longer than the others. this helps to keep things straight as well as the tailwheel or "rollinglandside".
Another thing you might look at is the tractor, do you have any sort of stabilizer or anti sway bars on the tractors 3p.h. lift arms, sometimes these are chains. These help  prevent the plow from tracking too far to the left. When you pick the plow up on the 3 point hitch, try pushing sideways on the plow, you should have almost no side to side play.
 Another thing, if you have a 3 point hitch plow, is the horizontal bar on the plow that the lift arms attach to perfectly straight, or is each end of the bar offset from one another? This bar is bolted into position, but the bolts canbe loosened so that the bar can  be rotated to adjust for the left to right travel. This is what the lever and sector that I spoke of yesterday does for you.
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