2 things to consider.

1) The distance from the front tire centerline to the back side of the nose on that 56 sheet metal is pretty tight, and that front Dakota crossmember and the sting pockets on that Dakota & Durango chassis sits pretty far forward. My concern would be keeping the front crossmember behind the sheet meal with the cab shifted 2" toward the rear.

2) Those large rear fenders on they 56 sit pretty far back on the bed sides already. Moving the fenders 2 more inches rearward is going to put the rear fenders very close to the back end of the bed. That might make the bed look really long.

Two inches doesn't sound like much, until you are looking at it in person.

Pictures are stock 54 sheet metal, the only difference between the 56 and the 54 trucks is the area around the windshield. That 54 was front frame clipped with the F,M,& J body subframe, with the stock 54 rear frame.

My 49 is a 4x4 and there are enough sheet metal differences between the 48-53 and the 54-56, its not an accurate comparison.