The slant or the 318

Enjoy each car the way it is. The slant can be fun to do. Equip it lightly with some parts and enjoy it.
The total cost of a V8 swap is expensive. A swap is best left for those who must have it or desire a seriously pumped up ride/engine for there car.

The swap mounts for the Duster that will still use the /6 K-frame are about $40 dollars or so from Trans Dapt.

While you can drive the car with a simple swap without upgrading another other part, I would certainly suggest a Radiator and brakes from drum to disc up front and larger drums at the back. Also, if you have the 7-1/4 rear end, a upgrade to a 8-1/4 for any tooling around is a good idea. You'll brake the 7-1/4 pretty soon if you mod the car any little bit.

You can skate by with the OE /6 suspension, though I would not recommend hot dogging around town with it like that. (Been there done that, not a great idea)

The /6 trans will not bolt up to a small block. You'll need a small block trans. Then converter, shortened driveshaft if you change trannys from a 904 to a 727, shifter, linkage, the list goes on.