Weirdly if you do sell the 261s i may be interested!
I've got 313s on my 130i and i'd rather sell it with 261s or 208s and then sell the 313s separately.
I think 20mm spacers would have their own bolts to attach to the hub, and then you'd bolt your wheels to the spacers. OR you'd need bolts which are 20mm longer...
Spacers around £50 a pair, long bolts around £1 each?