Is you're car finished? There's an old trick where you split the rear tubs and move the inner over and inch or so. I'm not sure how much you can do it and still retain the back seat, this is a mod I'm going to do on my '65 coupe. You also have to modify and reattach the trunk hinge supports. The outer tub stay attatched to the wheel opening and isn't modified, though you might want to roll your fender lip. It sounds like you want as wide a tire as you can fit and this is a way to do it. I believe the leaf springs can stay where they are, though you might have to take a grinder to the edge of them where the fat part of the tire is. Last I saw this done was on a '68 coupe with slicks, but it didn't have a back seat and the wheel wells were also modified with a BFH (Big Ford Hammer) to help square them up a bit to fit the tall slick. Your wheel well tubs have a neat little step in them and a vertical flange. The verticle flange is how they're spot welded together. You remove this and take a flat piece of metal and weld it to the steps from the tire side of the well. If you're looking at it, it's pretty self explanitory.