CHEAPest way to convert rear axle bolt pattern to big bolt pattern???

I am not positive on the 60-62 "A" body rear end assy, but I am positive on the 63-76, "A" bodies. The complete rear end assy can be swaped, they are the same width, over all, and the same distance between the spring perches. If replacing a 7 1/4 with an "A" body 8 1/4 or 8 3/4 rear assy, the driveshaft must be shortened, 8 3/4 a greater amount then the 8 1/4.
PS: The "A" body 8 3/4 never came as a LBP.