Technical Mounting rear spring perches for model a

I am getting ready to Weld Spring perch brackets on a newer rear to mount the Model A buggy Spring. I was wondering how to space those brackets do I just go approximately The width of the Spring or should I spread the Spring out some?? Many thanks

 

Stock model A spring fits a 49" perch centers. A model a spring needs to be spread alot.

 

Sounds right to me. However I couldn't get under mine.
I measured spring eye to eye at 46" and upper ( 2" ) hanger bolts 47" ( thru the trunk )

 

You need to have an allowance for the shackles, so the distance is a bit longer than the uninstalled spring. The number that most often comes up is 49" - 49.5". When you install the spring either disassemble the spring and install the main leaf, or have a purpose built spring spreader and be careful.