From memory a 25mm spacer clears them .... Or was it 15mm ?
But dont think the 350 spacer saver clears the front calipers whatever ..... Then you have the issue of swopping the rear tyre to the front and putting the spacer and spare wheel on the back ......
The problem then is that your 285 tyre and offset 20 rear wheel will make a right mess of your front arch due to the lowered suspension
I was going to buy a extra 19" rota alloy , whack a cheap tyre ( 245 section ) and store it at home .... Worse case , get a puncture , call wife to bring spare