I put a 12 bolt rear end in my 50 and used the weld-on spring perches from ClassicParts.com, and used the hole located near the end of the perch (as opposed to the center hole) on the head of the leaf spring retaining bolt, and it located the axle exactly 116" from the front axle.
Carl

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission