need help picking Kumho MX sizes!!

these are going on stock 01' C5 wheels, NOT Z06! whats the biggest i can go?
i was looking at the 295/35/18 for the rear, and the 255/45/17 for the fronts. will those fit fine the stock rims? i do push the car hard, and dont want any problems. would there be a problem having the 35's in the rear while having the 45's in the front?
also, is it really worth the extra money to go a tad wider? or should i just stick with the 245/45/17 and 275/40/18?!
sorry for all the questions, just trying to get these ordered within the next day or 2!! so any help would be appreciated!
thanks,







