Technical Spring Mounts on 9" Ford

Is there a specific distance apart that the spring mounts need to be when using a 9" Ford reaer end under a Model A with the original spring with reversed main leaf?

I have been given dimensions of 46"-46.5" and also 49".

Is there a right answer?

Thanks,

 

Center to center on the Model A rear end is 49". Difference in length between original and reversed eye is a little longer so you might consider that.

 

Fordors:
Thanks for the reply!