There are adapters/spacers available which allow Late RR wheels to be fitted to Defenders (Diffy?) and if I recall correctly they are around £40-£50 each but I stand to be corrected there.
Try Simmonites, I think they sell them.
maybe you could buy just 1.
then go to a local machine shop and pay one of the turners to nock you up the rest.
it will be cheeper and they will probably be of a higher standard