You'll be fine with 225/40R18 tires.
I would recommend at least a 225 tire width, to get the rubber at least out to where the body of the car starts.
If you are planing to keep your car stock or only drop the suspension from stock to nor more than 1.8inch, I would stick with a a 45 aspect ratio, to help fill up the wheel well. Anything larger on a dropped celica could rub. Anything smaller on a small to non-lowered celica, will leave a huge gap between the tire and wheel well.
REFERENCE EXAMPLE: 265 / 70 R17
265=Tire Width / 70=Aspect Ratio 17=Rim Diameter