largest tire on 14" rally wheels

a 10.50 - 12.50 will mount on a 6" rim. As any guy with a dually will show you that.

225 to 245 is 20mm or 2 cm. Not a big deal.

That's nearly 0.8". The general recommendation is around 2" wider than rim width for the tire. It may not seem like a big deal, but the wider tire puts more/different stress on the sidewall, since the sidewall is taking the weight of the car. You might be fine, but, then again, you might not be. Sidewall failure can get nasty.