STEERINGSince original Ford steering boxes usually wore out long before the rest of the car, rodders in the '40s and '50s resorted to using '40 Ford boxes, and then later F-100 units as replacements, which were basically more heavy-duty versions of the system already in place. With a dropped axle, split wishbones, and a stock or F-100 steering box in place, the rod that runs from the pitman arm to the spindle (called a drag link) was forced to operate at an unusual angle due to the geometry change. What would happen is that, as the frontend went up and down, it pulled the steering rod back and forth along with the rest of the suspension, causing the wheels to turn on their own, which is called bumpsteer. Essentially, chassis flex could make the wheels turn without any driver input on the steering wheel. This problem became even worse in the late '60s, when Mustang steering boxes began to replace F-100 units. Due to the way they were designed, the Mustang boxes needed to be mounted with the pitman arm pointing up rather than down like a stock box, so the drag link became very short. The shortened drag link further accentuated the bumpsteer problem.
the link for more reading:http://www.rodandcustommagazine.com/tec ... index.html
or mount the steering box in closer to the straight axle 
