Technical Rear T spring install width

I have been looking around on here and maybe im not very good with the search bar. I am building my dad a 27 coupe with a model A frame. In the rear we are using a model T spring for a drop, but I am using a 46 open driveshaft rearend out of a ford truck.(rearend had leaf spring perches right now) I am going to stall the rear spring forward or behind the rear end to get the car to ride lower. My question is how far apart do my brackets need to be so my rear spring with shackles is installed correctly. I know when it is mounted on top a normal A and T spring have to be stretched pretty far to be able to install them from what I read.

Thanks

 

I just measured the rear spring perches on my stock T. They are 48" (or maybe 48-1/8") c-c.

 

I set mine up with the shackles straight out, then once the weight of everything was added they set almost perfect.

 

the hot rod site seems to be down, at least for me but there is here i got mine. its a t spring profile but an a spring width. the rolling bones guys use them also.
http://www.t-springs.com/

 

Awesome thanks guys! Huge help