In theory you could use any rack but it needs to have the right distance between the knuckles - ie the swivel where the track arms joint to the rack. If you were to draw a straight line from the pivot point of your top wishbone to the pivot point of your bottom wishbone the pivot points of your rack need to lie along that line. As the distance between the top wishbone pivots is wider than the bottom ones the distance obviously changes with the mounting height of the rack. If the three pivot points are aligned correctly bump steer is eliminated. The other thing is to try to keep the lower wishbone arm and the track rods at a similar angle - this also helps eliminate bump steer. In Chris's book he shows a diagram of this and gives in ideal rack length of 533mm - this is the length from the centre of 1 pivot point to the centre of the pivot point on the other side.
Other considerations are the number of turns from lock to lock, the mounting methodology and the size of the spline on the input shaft.
Hope that helps,
Dave.
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