largest tires that will fit stock 68 Barracuda notchback

I was running a 275/60-15 on my Barracuda without moving the springs. The backspacing was 4". It was plenty clear on one side but very close to the springs on the other. The best thing to do is actually get both wheels off the ground and get a true measurement on both sides of the distance from the drum face to the leaf spring. From there you can decide on rim width, backspacing, and overall tire width.