These are really cute! You've run into the same problem many have experienced...the stitching draws the fabric in some cases. And the fabric is sometimes not as stable as we'd like since it is not as tightly woven. Those designs with straight borders are tough for just about everyone. Since that border is not that important to the design, I'd be for taking that stitching out and trimming the block again...leave the block as large as possible but make sure the unused printing will fall within the seam allowance. For future projects of this type, try to avoid straight border designs, stiffen the fabric a bit before stitching, and/or baste the block to another piece of white fabric that has a tighter weave to reduce the degree of distortion.
Hope this helps. Those tractors are really neat!