The factory heat exchanger system is pretty simple, yet effective. I had plans to use the factory stuff too, and still may. For me, it all depends on how well it fits into the front of the car, not as well as the Challenger I suspect. In the meantime, I'm researching good quality aftermarket stuff that may fit better and possibly be even more effective.
One of the things I'm also researching and drawing in my mind is the use of the killer chiller system similar to what's used in the Demon, or an ice tank, that I'd only use at the track. I'd have a diverter valve that would divert the coolant through the tank (after the heat exchangers) before re-entering the blower.
The first HC engine in an A body (that I know of) is a guy here in the Bay Area who had a really nice, custom '68 Dart convertible built by Dominator Street Rods. It was first built with a 440 and toured around a bit and was shown in a Mopar magazine or two. He took it back sometime last year and had them put a HC engine in it. A friend of mine saw it at Reno's Hot August Nights last year. I've since heard it was seen at a local (to me) Cars and Coffee meet one weekend. I want to hunt this guy down and check out his Dart. It's pretty nicely done from what pictures I've seen. From the pictures however, I could not see what mods they maybe performed on the firewall.