3.9's are pigs on fuel. Look at the epa ratings, they're only rated at like 1mpg better than the 5.2's when in comparable 2wd dakotas! Trust me, my parents had a 96 dakota, just a plane jane reg cab 2wd truck and it was a pig on gas. No power either. Ran good and all but sucked as much gas as a V8. That v6 dak got about the same mileage as my 74 dart /6 with bad blowby got.

If you're looking for something nice and driveable, you're looking in the right place. If you're looking for something with some decent power and decent fuel economy, you're way way off!

Neither the 5sp nor the auto trans from a dakota is going to fit in your car without some serious floor surgary, just so you know. IMO if you could find and build a 273, you'd probably be able to get the same or better mileage out of it with more power.