I used 275/40/17's on my factory 17x8 wheels. 285's should be fine on a 9". I've got 275's on my wifes 9" cobra wheels and it could easily stand one size bigger.
you can go to tire company websights or www.tirerack.com and see what the recomended rim width is. I believe 285's should use a 9.5 inch wide rim. They'll fit on a 9" but have a more rounded or balooned look and may not get the full contact patch and offer less traction than a smaller tire on the same rim.
The previous poster is spot on -- contact patch is limited by rim width. You may physically be able to mount a much wider tire on a particular width wheel - but that doesn't mean it will be optimal.
A good rule of thumb is that you don't want the section width of the tire to be any more than 2-2.5" wider than the wheel width. This will allow the tire to put it's full contact patch on the ground. The section width is the first number in the tire size in millimeters. So a 275-series tire has a section width of 275mm. Divide it by 25.4 to convert it to inches -- 275 section tire has a section width of 10.83". Subtract 2-2.5" from that and you have the range of widths that will be optimal. The 9" wheel should work fine - for the 275 or 285 section tire.
....and whether or not it will fit on the car is a whole other issue; test fitting is about the only way to be certain. That's why it helps to have a good relationship with a good wheel/tire store -- so they can help you make the best guess, and then help you test fit.
I just went through this on mine -- VERY limited room in the back and I wasn't sure that I could go up in size. So we test fit a larger tire on my wheel just to be sure. It all worked out -- but I wouldn't have known for sure without the test fit.