The first thing I would suggest is to get new pads and install them. Floating calipers change position away from the wheel as the pads wear, it would be rather inconvenient to get spacers then have them rub again with the next brake job.
I am not a fan of grinding the calipers for clearance, a lot of people do it and get away with it but, it is not the best way.
The best way is to get wheels that work with your car; spacers would be the next best solution.
Using washers to help determine the correct spacer is not a bad idea but, do not drive the car with the washers, wait until you have proper spacers.
You want 3mm to 4mm of clearance between the wheel and caliper.
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