Small changes to the lengths of the wishbones such as this won't make much difference, although if you sort out the lengths I'll feed them into my suspension program just to be sure.

Although the differences in track are a good guide to wishbone lengths, because of the possible differences in wheel offset, it's not definative. Track is measured from the centre of the wheel and tyre, therefore the offset of the wheel affects the track. Two cars with the same track may have very different wheel mounting face dimensions

The important dimension is the wheel mounting face to wheel mounting face dimension.

Other things to consider are:-

I believe that the BMW hub is part of the rear suspension trailing arm, this will need machining or replacing to fit the Roadster type upright.



This is the E30 rear hub.

Are the calculated positions of the hub fixing bolts in relation to the upright within the area of the Plate as described for the Sierra version or will a larger plate be required?

Will the brake set up be suitable?


The ideal way to sort this would be if anyone has engineering drawings of the BMW hub or access to the parts for accurate measurement.

Cheers

Chris