71 dart swinger tires

I own a 71 Dart swinger did a 4.5 bolt pattern conversion. With my current tire package i have maxed out all the room the factor wheel well has to offer.
I run 18x8 centerline stingrays with 5" back space. With 245/40/18 nitto555's
Now this is the same diamiter of the stock tires (25.5"), But with the 5" back space it will fill in the wheel well all the way. I have about half an inch to the leaf spring and a finger in between the fender. Now the front I got very lucky on with the turning radious. full lock right or left the rear of the rim just misses the frame by half an inch. So 8"rim will max out all the room but you need a 5" backspace. Now for your current sizes, The rim you have should have is only a 14x6 rim. 215 is the widest you can fit on that rim.