The guy next door to my shop manufactures rack and pinion systems for late '50s-early '60s full-sized Fords/Mercs as well as little Birds. In fact he just finished his prototype for '67-'70 Mustangs/Cougars that will be available shortly.
I posed the same question a few months ago and he said that the effort was about the same for both. If there is any difference at all, it might be slightly more difficult with the rack and pinion.
Ive never used on on a cougar, but my old hot rod truck had Mustang 2 susp and a power rack. I had a ps fluid leak for awhile and I just bypassed the PS pump and used the rack as manual. It steered just fine and only took a little more effort than when it was powered. So I cant imagine a manual rack being any worse than a stock manual box.