Dunno if it helps you or not, but you might investigate the 'Sportspack' option from England - I know they were fitted with 12" rims & sportspack arches, which I think are 3.5" wide. If it is OK to get sportspack arches as a factory extra in Pommieland, then you can probably argue that you can put them on your car here, & still be legal.

You better talk to an engineer . . . . .
Oh, BTW - Make sure that "Standard" is actually standard for your car - Later cars had arch extensions to make them legal, so I would assume that they had a wider track too.
