Dodge Dakota V6 flywheel on a LA

3.9 and 318 are both internally balanced. You should be good to go, just make sure the 318 crank is drilled for a pilot bushing/input shaft clearance.
Edit: As long as you're not using the computer controls on the 318. That flywheel only has 6 pickups for the crank sensor. If you're carbing it, you're good.