Moving the rear forward/back to center in wheel well

[siggyfreud]

Hey all,

I'll be welding in a new rear suspension setup in the future. I don't have access to my bed currently. Was wondering if any knew how much one would want to shift the rear forward or back on a leaf spring truck to better center the rear in the wheel well? I'm working on a 66 GMC. I currently have it attached via leafs, so if I knew the change it would be fairly easy to shift it forward or back a bit to better center it.

Appreciate any input people have.