What tolerance is required for a spring relocation?

I am doing a spring relocation using the US Car Tool kit. As it sits now, I have the boxes within an 1/8" corner to corner. That's from the torsion bar mounts to the outside bottom spring mounting holes on the boxes. I also have the boxes parallel to each other. I used a 5/8" threaded rod running all the way through both boxes to give minor controlled movement to help get the boxes parallel. My question is, how close do you guys think it needs to be? I think I can get it within a 1/16" corner to corner, maybe a little better. I just need to know how far I ha ve to take it. Thanks for your input.

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Did you have to cut the threaded rod out once you welded in the boxes?