i have a question about lowering the rear of my 04 sedan.

little info -

right now i have 18s on the car that i pulled off a coupe. and they stick out in the rear on my car about 3/8 inch. on big bumps if im going someone fast the rear end drops down far enough to hit the tires. the tire size is 275/35/18. on a 18x10.

actual question -

is there a spring for the rear that is really stiff or something that i can use to be able to lower the rear and inch and not let it drop down enough to hit. because if i lower it an inch then its going to hit even more. so i would need something stiff. or does anyone have any other ideas? and no i dont want to have to get another set of rims. i cant afford another set