33gtst guards are fairly generous when rolling,
I'd say you would easily be able to get the 265's without flaring as long as you pump the whole guard out a bit.
need to roll then set the guard roller high up in the guard and use it to push the whole area.
I had 18 x 9.5 +12 w 235/40 and tyres tucked a little with almost 0 camber and 0 scrub from pumping the guards
this is slightly more aggressive than what you are looking at with the 265's and with 285's the outside top edge of the tyre will sit in approximately the same spot, but with a slightly bulged sidewall.
reference:
http://www.willtheyfit.com/index.php?width=235&aspect=40&diameter=18&wheelwidth=9.5&offset=12&width2=285&aspect2=35&diameter2=18&wheelwidth2=10&offset2=38#content
so yeah, for the 285's it's possible without flaring but you would need a serious pump