I'm doing a 14B swap right now.
I recently picked up this little jewel.
Since not all 14B's are created equally, what you'll want to do is to measure the center width of how wide you are setting the spring hangers at the frame (if stock width, so be it). Then take that width and replicate it so that your spring perches sit correctly on the tubes equal widths from the backing plates.
In other words....lets say your center measurement for the width of the spring/shackle hangers is 42". If the width between the 14B is 52" (backing plate to backing plate) then you would set/center them 5" from each side while taking into account perch width etc.