I'd go with the magnum- The heads will flow better, lighter package and roller cam. Pop a used roller cam in off the Dak boards and roll with it.

The 5.9 in my Duster hauls butt.

If you are going to start tossing new heads and stuff at either the picture gets more murky. I'd still go with the SB unless you are going to stroke and then I'd run the 383 (496 yo!)