leaf spring shackle width?

i'm going to run a mono leaf with a 4" dropped front axle.what i'd like to work out is the centre to centre measurement for the outer most shackle mount points.
i am going to mount the spring behind the axle with a tube shock in between these(as in the photo i robbed from the internet).
can i just put my spring under the press load it up to the approx weight(what weight will i be looking at for a fenderless small block chev powered 34 ford steel pickup?)measure it then add the width of the shackles mounted on a 45 degree angle to get this measurement?
the axle is a superbell '28-'34 ford 4" drop that measures 34 1/4" centre to centre of perch mounts.
the spring of unknown brand-mono leaf that measures 30 1/4" approx.

any help appreciated.

 

speedway motors.com has a chart depicting the mounted spring length/perch spacing you need on their site or at least saw one in their catalogs.....

 

thanks i will have a look for it.