If you want a modern Hemi, I think I would start out with a running, driving Hemi Durango. Then modify the 55 sheet metal to fit. You would be way ahead of the game if you use the firewall and the front floor pan from the Durango. The box is going to end up being about 10" deep, so I hope your not intending on hauling much.
Its not a simple nor an easy job, but makes a fun truck in the end.

The next option is to use a Dakota front clip, (joined behind the Dakota trans crossmember). Then pick up a Hemi & its trans. www.hotwireauto.com makes a stand alone harness (you need to connect to 4 wires from your ride's harness) to make the Hemi run with the EFI, its not cheap, but it works well. Gene