I'm going to disagree on the 3.9 actually, and here's why-
For a V6, they get lousy mileage. We had an '02 Dakota, just a standard cab, no feature 3.9 with a manual trans. It never got any better gas mileage than any of my 318's, including the carbureted (4v) 318 auto in my Challenger. It does, however, have less power. So, in my opinion, I'd just get the 5.2 (318 ). Same mpg's, more power, just as, if not more, reliable. Obviously, its not a straight across comparison (weight, gear ratio's etc), but a '72 Challenger with a 318 and an automatic should not get equivalent mileage as an '02 Dak with a 3.9. I actually have gotten better mileage on the freeway with my Challenger than we could get out of that 3.9.
Strike two was the issues that we had with it. Now, I won't say they're all bad, we may have just gotten a lemon. But in less than 30k miles we had a "knock" show up that led to the engine being replaced (under warranty fortunately). Less than 500 miles after the new engine was in, one of the heads cracked. So, two different 3.9's with low mileage that bit the dust.
My old man has a quad cab '02 Dak with a 5.9. Love that truck. It has over 160,000 miles, very few issues with it, just minor stuff with the electric door locks and stuff. They're good trucks, but I wouldn't get one with a V6. The loss in power isn't worth the increase in mileage, because there really isn't an improvement in mileage.
If this isn't a primary vehicle for you though, why not go pre-smog? I've seen several sweptline Dodge trucks in my area recently, all are under $5k, and a couple of them are pretty nice. Just a thought. I daily drive my Challenger, it literally is the newest model year car I own. I don't have a huge commute, but if you have fall back vehicles it might be worth considering. A nice 318 sweptline would be a fun truck.