Better get the RAM frame mount brackets as well. Hopefully you can adapt them onto your 56 frame and they will still line up with the Ram brackets welded to the Dakota rear end and you can use the Ram linkage.

Don't know how well just using the trailing arm mounting brackets off the Ram axle are going to work if you don't use the rest of the Ram trailing are set up. At least in theory, the Ram parts could be mounted with everything at the same time to assure alignment of the parts before permeant welding occurs. I would think a complete mounting system from an aftermarket supplier would work better then trying to adapt some of the Ram parts to a different aftermarket's designed parts will work out.
I have no experience with rear coil spring and trailing arm suspension, but mixing parts from one company to a different company doesn't sound like a good process.