73 12 bolt rear for a 67-72 trailing arm set-up

[Z11WEAVER]

Can someone please tell me the width measurement of where to weld on the axle seats? The center housing edge lip width where the axle tube slips in the center chunk is exactly the same width as a correct 67-72 rear at 16.5" wide, from edge to edge. How much futher out from the edge of the center housing do the seats get welded on to the axle tubes? My guesstimate is 12.5 " center mount hole beyond the center housing edge??? Total of 41.5 center hole to center hole to mount the trailing arms to???