Looking to use dakota springs in the rear for a soa stretch. Question is what shackle to use. I welded a piece of 2x4 tubing to the bumper with bushing welded half way up the back side. How do I figure the lenght of shackles need to work?

Aim for a length that puts your shackle at 45 degree angle.

You could start by searching, there are about three threads about leafs right now.