Calculators your friend
http://www.tyresave.co.uk/tyresize.html
Liam I'm not sure that is the right calculator is it for the question?
I would try the one on this link, 2/3 of the way down entitled 'Ideal Rim Width'.
http://www.carbibles.com/tyre_bible.html
The answer to the original question is both However, Liam's link will tell you what tyres you can use by comparing with stock and not upsetting rolling radius and the answer is either a 275x35x19 or 285x35x19 will work on 19" x 10.5" rims although the 275 will appear more stretched than the 285
I may be wrong but I've seen it said elsewhere that 285s on 10.5" rims may have some rubbing issues so that the arches need rolling. That's certainly the case if your planning to drop it at all.