3.9 into K-frme

Not sure if the truck pan will clear the QA1 K frame, I would say probably not. My guess would be that you'd need to do something like a HemiDenny K frame with a conversion to a steering rack if you wanted to use that truck pan, the stock steering will still be in the way with a QA1 K. You'd probably be better off making up a custom pan, as I'm not aware of any mid-sump pans for the 3.9, and aftermarket offerings are pretty much nil.

As far as the 3.9 goes, I hope it works out for you. My own personal experience with the 3.9 in a 2002 Dakota was that despite having significantly less power than a 5.2, the gas mileage wasn't significantly better. Not enough to make it worthwhile. They were only EPA rated at 14/20 mpg for city/highway, and I found that I pretty much never got to that 20 mpg number. I've gotten 17 mpg out of the carbureted 318 in my Challenger. I would hope the 3.9 would do a little better in a lighter vehicle like your Dart Sport, but keep in mind the Dak had a 5 speed overdrive manual to get to those numbers. So, maybe not.