Wheels
Lay the Tire and Wheel down on the Floor,Inside up.Lay a Straight Edge on the Wheel Lip,take your Tape measure and measure to the Mounting flange.If you have say 4"s?Then i would go with 7"s that way you still have 1" inside the Bed lip.If you would want 2"s to the Outer Lip then you Need 8"s of backspace.
Hope that HELPS?
Later
G 8)