Someone else who knows for sure, chime in, but I am pretty sure my axle is centered on the leaf spring. So measure where center is, from leaf spring mount, to leaf spring mount. I am using a 55 1st series rear end. I had new springs made for this purpose. Its been years or I could be more definitive. I am happy to measure for you if nobody else responds.
Stock 47-53 (I don't know about 54) leaf spring "center" bolt is not located in the center of the springs. I believe it is off-center due to the funky torque tube axle-to-spring mount.
Carl
