What I'm reading is rack and pinion is not recommended unless you do a full frame (like a Schwartz g-machine frame), and there's no reason for a tubular k-member unless you need clearance for headers.
So seems like leaf springs and stock k-member suspension is the way to go for a street car.
Last time I drove a '60s muscle car was a '69 big block Camaro, it felt like a pickup truck.
I wrote a whole book on the subject so I'd say start by reading the book. The short summary is that Mopar musclecars were designed 50 years ago using 50 year old tools and techniques. The early unibody designs are flexible which makes the car feel funny in a corner. They didn't feel funny 50 years ago but now that people have 20 years of experience driving solid cars a muscle car feels flexible when you go around a corner.
If you are willing to cut and weld on a musclecar then add subframe connectors, torque boxes, inner fender supports, additional cross members, shock supports, etc. Once you have the chassis as stiff as you are willing to make it then do a total rebuild of the suspension and use high quality shocks (Koni, Fox or Bilstein) use modern upper a arms (Firm Feel or Hotchkis) use high rate torsion bars, use a big anti-sway bar and have the steering box rebuilt and tightened up (Firm Feel).
After you do all of that the car will handle okay. Kind of like a new Honda. Not like a new BMW or Porsche but well enough that you'll feel comfortable driving it at 50 mph around a curve that says 35 mph.
