rear suspension mounts

[deezee]

It sounds like you need to lay your chassis on a proper flat surface and work out a proper reference. I wouldn't be wildly worried if it was ±3mm on mounting positions (my front upper wishbones have a 3mm packer on them) as its not going to cause any problems. But if your suspension is 6mm out, I'd want to be sure what I'm taking my measurements from was a good reference.

If you can get the mounting points to the correct position, relative to the front suspension mounts, I wouldn't be too concerned. Normally you wouldn't have to worry about that, but by the sounds of it, your chassis is going to need checking.