Anybody running 17" Cobra Bullit rims on a Duster

Hey guys just a quick question. I was able to pickup a set of 17"x 8" mustang wheels from the yard and I need to know what size spacer I'll need for the fitment on the rear only. Tire size is 255/40R17 and at the moment the tire contacts the leaf springs on a 1965 Barracuda with a suregrip 7.250" rearend.

Thanks for your input..