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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.
great minds think alike. if I were to use a 3.9, I'd use KB167's at 0 deck, and have the cam reground by bullit with the same lobes I'm using in my 318 (HR259/316) and toss a V8 TB on it.
one downer, IIRC, the 3.9 only has the newer truck mounts, not the old LA ears on it, so you'd still have to get creative on fabbing motor mounts.
I'd probably look at a 3.5L out of an LX. heck, even a 3.8L out of a wrangler in a light A body would be decent--our 4400 lb caravan gets pulled around by one just fine, would be downright peppy in a 1300 lb lighter car....toss it in front of an A833OD or AX15 and 3.21's
1976 Spinnaker White Plymouth Duster, /6 A833OD
1986 Silver/Twilight Blue Chrysler 5th Ave HotRod **SOLD!***
2011 Toxic Orange Dodge Charger R/T
2017 Grand Cherokee Overland
2014 Jeep Cherokee Latitude (holy crap, my daughter is driving)
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