18×9’s on a 68 coupe
measure twice.. buy once..
place masking tape across sidewall of tire to indicate the lower lip of the rear wheel opening.. measure down from the front wheel well opening to the center of the front wheel if the lip of the wheel opening does not come down over the top of the tire..
support car with jack stands under rear axle.. see if the wheel is in the same position.
you can use something like feathering disc adhesive to glue florist foam blocks to lightly the blocks to the tire. this allows you to carve and trim the foam till it will just clear the inner wheel wells.. while you rotate the tire.. you might want to add some sand bags or heavy weights to the trunk .. to duplicate hitting a bump..
a friend used that idea.. and actually used a jack to spread his wheel wells slightly on his fairlane to wedge in some i think he went to 15×10 rims in the back