Its not a few mm though Paul, its the thickness of the adapter plate the OP has made to take up the slack.

As you know they tried every trick in the book on those cars, making the bush thinner and shortening the platform would reduce the centre of gravity by the thickness of the adapter the OP has made. One of the biggest complaints bout the 500 was it was top heavy, they would have tried anything to get them lower.

I am trying to find a rational reason why the solid mounts are much thinner than the rubber ones. Its been done for a reason, otherwise they would be the same thickness.