I've screwed them to the garage floor. This means they cannot move around when positioning the van as others have suggested happens. I can also get the van in the same position every time and I do not need a second set of eyes/hands.
For anybody who is interested, the distance between centres of the wheels when positioning the savers is 67 1/4" or in correct units of measurement 1,708mm. If you have an odd bit of plywood then this will help geting the two parallel and in in line. If not a few sheets of newspaper taped together would equally do the job.

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Colin