Something is wonky with your tires because a 245/40/18 and a 275/35/18 have the exact same diameter and therefore will have the same sidewalls. You sure those sizes are what you have?
Go on tirerack and just get a staggered setup that has a matching overall tire diameter or really close, like within .1 or .2". If the diameters are the same and the rim opening is the same then the sidewall height will be the same too.
edit: Also the stock 25.7" tire diameter for a 94-04 car looks like a damn roller skate wheel. I suggest going up a sidewall size on the front and rear or even two out back. I run a 235/45/18 up front on a 18x8 and a 275/40/18 out back on a 18x10. As you can see I have room to go up a full extra inch probably because that's only = .5" tighter to the wheel well.
There is also no reason to go bigger than a 275 or 285 on a 10" wide rim. My 275s bulge out just very slightly which is optimal.