1st gen Dakotas with V8s weight about 1000lbs more than a 318 2bbl a-body. The 3.9 V6 is for MPG than the 318 but not much in a dakota, my 2000 4x4 ex cab gets 18-21 on the highway and tows just fine as long as you don't expect it to pull a car over a pass @ 75 mph. A 3.9 magnum would move an a-body handily, a 4cyl a-body will get better MPG than a 4cyl dakota because it is still around 1000 lbs lighter. Do not be afraid to try it, I would run a 3.23 gear and an a-500 and I would bet you could get 30 mpg and handily kill an average slant six in a drag race.

Now if you want to really go all out on it, get a 5 speed to swap in and install a set of 6 KB167 pistons and get the cam reground with slightly more lift or install some 1.7 roller rockers for nearly the same effect. Put a V8 throttle body on it and fill in some of the plenum area in the beer barrel intake. You will probably need some custom headers for the best MPG.