Well, judging from the description, they took bumpsteer at least into consideration, unlike Flaming River....
"Update your classic Mustang's steering this rack "& pinion kit. This is a manual rack & pinion that is custom made to match the stock steering geometry of the Mustang. This manual unit comes in a ratio that greatly improves handling, steering effort, and road feel. No bump steer! This is a bolt-on kit, no drilling or welding required. The kit comes complete with the custom made rack & pinion, mounting bracket, outer tie-rods, shaft kit, column bracket & bushings. That's everything that you need to install this kit with the stock steering column."
In case of a power steering rack with the tie rods attaching at the ends (like Flaming River), you need to find room for both travel for both the rack and the power steering piston. On 65-66 Mustangs, the distance between the tie rods is ~18.5" and on the later cars it's even less, so fitting everything in between with some decent travel is difficult if not impossible. However, since this is a manual rack, it's easier to make it short enough to match the original steering geometry without going to a center take off design (like Randal's and TCP).