According to the offset calculator at the top of the wheel and tire section, the difference in rolling diameter between your 255/35 series tire and a 225/40 series tire is 0.235%, so in that aspect you should be fine.
However, I don't know if I'd personally risk stretching a 225 series tire on a 18.5" rim. It will have a stretched look and provide less protection for the rim face. I use my stock rims (same sizes as yours) as my winter wheels, but my winter tires are 225 front and 255 rear. While it may not be the optimal size for snowy conditions, it still provides enough traction for safe driving. And there are only a few snowy days per year where I live anyway. The rest of the time it's just cold, so I prefer a staggered tire for more grip.
Last edited by Patrick719; 01-14-2012 at 04:06 PM.