what size wheel

Ok so all the body work is done on my 66 dart. Everything is in primer and getting ready to lay paint. Here is my current issue. I put an 8 3/4 rear from a 68 Dart GTS in it, did a moser Axle swap and a disc brake conversion. Now I need to get some wheels and rubber. I placed the jack stands under the rear end tubes to get the suspension at ride height. When I measure from the center on the axle to the bottom on the wheel well I get 13 inches. What I am trying to figure out is what is the largest tire I can get in the back of this car? Also is there are length difference on the rear end from the original 66 to the 68? now I do not want to roll stretch or do anything to the rear fenders as the body is now perfect. I was thinking 17x8 or 17x9 with a 5" backspace but I test fit a 17x9 and it seems a bit too wide. looking for suggestions, but I am looking to get a 255/x/x back there. Anyone with any good idea's? Thanks in advance.