I have ran a vega cross throw steering on 3 rods that I have built, and have never experienced bump steer on any of them. I use an aftermarket pitman arm that measures 6 1/4" center to center, is perfectly flat steel 7/8" thick, and install the drag link rod-end from the bottom side. I fabricate my own Panhard rod, and as the other gentlemen advise, I set it up to run parallel to the drag link and tie rod when the front spring is under full loaded condition. I mount it to a bracket on the underside of the drivers side framerail, and to the batwing on the passenger side. I try to run 5 or 6 degrees of caster, (kingpin tilted towards rear of car at top), and whatever camber is built into a Superbell I-beam 4" dropped axle. The spindles I have always used are stock 40 Ford units with the stock steering arms, set up exactly as you have shown. I use an aftermarket Model A spring with reversed eyes, and "normal length" shackles. The steering boxes are mounted so that the input shaft (on the steering column side) is tipped up at an angle of about 10 degrees from the frame at the rear side of the steering box. I can't imagine why you are having a bump steer problem. The only thing that I can think of is this---do you have a panhard rod on the rear axle, tied to the rear-end and the passenger side of the frame? that would do it. (the rear Panhard rod should always be pivotted off the same framerail as the steering box is mounted to.