Well you can get a better steering ratio from a Borgeson steering box, they're 14:1. I run a 16:1 manual steering box on my Duster, and really it doesn't need to be any faster a ratio than that. Now, if you want to get rid of the on-center dead spot that every worm and ball steering box has, you have to go rack and pinion. But that's not a performance thing, that's a feel thing.
Not sure what to tell you about the headers with a non-mopar engine. The odds of something "off the shelf" fitting the swap probably increase with a coil over conversion, but that's a crapshoot as it's entirely possible nothing off the shelf will fit regardless. That's the downside to less common engine swaps, you're on your own for headers. A set of custom headers should set you back less than the difference between a coilover conversion and a well built torsion bar front end though. But suppose there's the engine mounts too, as Denny might be able to do that. What engine are you swapping in?
"Drive like a new car" isn't a fair comparison, IMO. There's a lot things that go into how it feels to drive a new car. Most new cars have independent rear suspension, first off. Not to mention traction control, ABS braking, and depending on the vehicle other computer driven handling improvements. Like "sport mode" that can do things like change the resistance on your electric power steering, or the shock damping for cars with magnetic shocks. Heck even raise and lower the ride height on some cars.
You're not going achieve most of those things with any aftermarket suspension, coil overs or otherwise, unless you're planning on grafting the body of your A-body onto a modern car's pan.
If you want your car to handle as well as a new car, that's achievable. Heck the Hotchkis Challenger pulled more G's on the skidpad than a modern challenger. And the Hotchkis Taxi lapped TireRack's test track faster than the 3 series Beemer's they normally use, with the same driver. The Hotchkis cars are still torsion bar/leaf spring cars, and quite frankly there are some parts out there now better than what Hotchkis offers. And not to brag, but my car handles as well as either of those Hotchkis cars. It's set up the same or better. But I can tell you right now, none of those cars feels like driving a new car. Performance wise they can compete, especially if you do something silly like turn off the traction control on your new car. But driving the old car to those performance numbers requires more effort and more skill to make up for all the computer driven goodies you won't have.