Astrofriend I hope you are successful in correcting for your 5mm difference. I have no experience with what you are trying to achieve but had some thoughts about the basic condition of the frame structure. It seems to me there must be a reference point or datum used for mounting the suspension system. If there is and are reasonablly accessable they can be used to determine if the mounting points are equal locationwise. This would help to establish if the frame structure has been twisted or distorted in some way. This could be why you need to shim. Just a guess.