Fuel probably wont fit. The slant runs thats in it. The 3.9 will have more power than a slant in stock form especially a Magnum. Plus its not cheap to hop up a slant unless you find bargins witch are getting scarce. The 3.9 can be mounted forward some. People have installed V8s in them so it would be slightly easier with a V6 and rear dump manifols would imo would be best. The shifter lever can be modified to fit. I think a wrecked Dakota can be had for $350 or less and thats cheaper than a 390 Holley 4bbl so price wise is a very gray area do to not knowing what a wrecked truck can be had for and I think it would be a great gas mizing cruiser