The 6" difference in wheel base is not a problem as you will need to make a new bed and mount the fenders where you want, or if you go flat bed(personally I like a PU bed)
as far as the track width goes a Dakota is a little wider than say a mustang II so it would depend on what off set wheel you run on the front and how low you want it to set, but if you go with a stock off set wheel and not a reverse wheel you should be OK.
just set the body in place on the frame, fit the front end to be sure the wheels are in the wheel well properly and start making mounts
then get or have some one bend up a bed and mount some rear fenders Ron...

I make it sound easy huh