I had two identical '68 Cougars one with TCP power rack and one with a newly rebuilt factory box. The only real difference was that the rack and pinion would not turn anywhere near as sharp as the stock set up. It is much easier and less expensive to just fix what you have.
I have heard the same thing also, you’re better off adjusting or replacing the steering box and/or new tie rod ends and so forth, or the Borgeson conversion, than to go rack and pinion.
I installed a Borgeson OEM-style steering gear, keeping the original-style (Though rebuilt) control valve and cylinder. Everything works well, no leaks. Yeah it’s a little bit loose compared to the electric rack in my 2016 Fusion, but it works well for what it is. Tight for a stock-style 69 Cougar.
Is that with the engine running or off? The power steering takes up about 1/2" of play when pressure is applied. With the engine off there is 1/2" play at the pitman arm, which translates into a lot of play at the wheel.
There is about 1/2" of play at the steering control valve that should disappear when the engine is running. If this is not true then you should find out why. Otherwise likely the steering box or rag joint or a combination of the two.