Good observation on the leaf still being in place and the tire appearing to fit - the tire is actually touching the leaf. With a leaf in the stock location I don't think you could run a 275 wide tire - especially if the car is lower than stock as the inner wheelhouse slopes inward an inch or so on the quarter panel side. I think you could maybe fit that big of a tire, or close, if you did edit the following:

1. Use a 2" wide leaf instead of a 2.5" leaf and offset it toward the center of the car. Giving you a bit more inside clearance.

2. Roll or cut back almost all of the fender lip on the quater panel.

3. The rim backspacing offset will have to be exact.

4. Nothing in the 4 link set-up interferes with the tire or rim at that width.

5. You will still have to move the inner portion of the wheelhouse over for sure or make some mini tubs.

No guarantee that all that will work but maybe.....probably a lot more likely with a 255.