You can absolutely put a 64-66 Barracuda in the same handling ballpark as a 2016 Challenger, and you absolutely don't need a coil over conversion to do it. With the right parts the factory based torsion bar suspension has just as good, if not better, suspension geometry than any of the coil over kits available in their off the shelf form.
Look at the Hotchkis Taxi. They did a tire comparison on the TireRack track, using the TireRack test driver. He put down faster lap times with that 4-door B body than he puts down with a modern 3-series BMW that he drives for a living testing tires. Same track, same driver, same tires, 4 door B-body vs modern 3 series BMW, with a driver that has way more hours in that BMW and has by his own admission not driven older pro-touring style cars. I think that says a lot about what these cars can do with their torsion bars.
Here's the video, it's little long. You can pretty much just start at the 3minute mark. At 4:30 you can watch the Hotchkis Taxi kicking butt. And at 6:00 min TireRack's professional driver goes into how the Hotckis Taxi is a full second a lap faster than the BMW 3 series, even though Woody (tirerack's driver) had never driven a "protouring set up car" before the test and has probably turned thousands of laps with that 3 series.
There was also a direct head to head with the '70 Hotchkis Challenger vs a 2010 SRT8 Challenger. The SRT8 lost on the skid pad to the Hotchkis Challenger, yup, torsion bar E-body. At the time the comparison was made (like over 10 years ago?) the Hotchkis Challengers suspension set up was pretty novel. By today's standards, with the parts that are now available that weren't when the Hotchkis Challenger was built, well, that build isn't radical at all- heck my Duster has a more aggressive set up now than the Hotchkis Challenger did then. This is the article, it's pretty wonky at this point as Edmunds must have changed their website several times since then. But the info is all there.
Now, that said, yes it can be done, but it won't be cheap to upgrade all the suspension, the chassis (with either suspension type), AND have the horsepower to make it close. And it won't be cheap regardless of which suspension you choose, there's a ton of work to be done either way.