Engine mount spacers

I have to notch the oil pan on my 360sb. In order to fit rack and pinion p/steer rack. I need to get around 10mm clearance between rack and notch tunnel. I have fitted a pair of 6mm spacers on the engine mounts , but still probably need about the same again to give the desired clearance. . Is 12mm spacer OK or will this cause unforseen issues??