Biggest Tire In Stock Wheel Well?

so i did some measuring on the wheel wells of my 68. i figured it is 4.25 inches fron the frame rail to the out side of the leaf spring and 14.25 from the frame rail to the outer lip. so that gives me a 10 wide opening. but for clearance in gonna stuff a 9" wide tire. as for hieght i think i can get a 27.5 or 28. then i put a straight edge down from the lit and measured to the flange and it was 5". meaning that the hub is dead center if the wheel well and that i would need like a 4.25 or 4.5 backspacing.