We've installed
this kit into one of our 427s. It's relatively cheap, but it lacks any speed control, so we also installed a switch to cut off the boost. Otherwise, the steering is really too dead at speed. The owner was very pleased at how it worked. With a little work, you could probably integrate a speed sensor to automatically disable the boost over 20 mph.
As has been said, there are more sophisticated (speed-controlled) boosters, but the price is way up there. "Adapting" a production electric booster is not easy, since they typically get several inputs from the car's main computer. That means, in a conversion, you have to "fake" some input through a separate controller.