Whats best wheel and tyre sizes you can fit on a 3dr sierra cosworth with AVO Coil Overs? i'm willing to cut back the body kit to help.

i'm running 17" x 7.5J wheels all round,front tyres are 17x35x 225 & rear tyres are 17x40x235, but would like wider tyres on the back.

Was thinking
front wheels 18" x 8.5J but what ET and tyre sizes?
rear wheels 18" x 9.0J but what ET and tyre size?
Or is it best staying with 17" ????????