By looking at your picture, I would think it would be a 5.5". Not looking at the body lift. What size tires are on the truck? Measure from the bottom of the frame down on level surface where the front bumper is mounted and let me know.
i think i have a 3 or 4" lift on my '77 but im not sure i have 33's on it and it's about 20". does that sound right?