The simple answer is NONE. By the nature of the beast the rack must be mounted to the front axle and the steering shaft is mounted to the frame/body. This means there must be a slip joint in the steering shaft that has to slide in and out as each bump in the road is passed over. The ONLY way the slip joint is aligned is when the pinion shaft is directly in line with the centerline of the shaft. Any above or below shaft position is putting the joint in a bind and grease can only correct for so much. If driving on only bowling lanes and pool tables I see no problem. They make a rack called Unisteer that uses cross steering which decreases joint movement but it's still there. If one wants a modern steering feel then look into having the box rebuilt by Lares followed by an electric power steering set up from a Prius. With p/s one can install increased caster shims, set the throttle and get into the bed while the truck goes 50mph STRAIGHT down the road. Dragsters are going 330mph with the same straight axle set up and radical caster.

Evan