i have a set of 18x8 et35 comp mo 6 spokes

i have 215/35/18 for the fronts
what size tyres should i fit on the rear? i don't want them to catch my arches but want them a little wider than the front and to fill the arches a bit.

the arches have'nt been cut or rolled but my body kit has been cut right back to sit flush with the arches.

what about 225/40/18?