I think I would start with some measuring on the 56 bed width at the rear end location. The 56 was a full sized truck, back in the day. When I put my 49 on the Dakota chassis, I had to cut the bed sides to clear the tires on the mid 90s Dakota rear axle. The bed side cuts were below the bed floor, mainly because the bed floor is pretty high on these conversions.

The Ram 1500 rear axle width might work out better, but then you would need to deal with the wheel bolt pattern. I wonder what would be involved with swapping the Ram 1500 front brakes with the Dakota front brakes? Unless you are in love with the 6 bolt wheels, the 5 bolt Ram bolt pattern probably has more wheel options, if changing the front brakes over isn't a major issue. .

Just something more to think about.

Pic 1) The inside edge of the tire rubbed on the bed side, so I cut out the diameter of the tire for clearance. The holes in the bed side are the fender mounting holes, the 49 rear fenders were just large single wheel trailer fenders, the fender mounting holes on your 56 will be spaced farther apart and more of a 1/2 oval shape. The top 3 fender bolts are above the bed floor line, the other holes are all under the floor.
Pic 2) Just more of an angle. You can see the bed floor height in this pic, the floor is the black metal inside the bed side.