What are your long term plans for the Duster? Only asking, because there are a lot of things besides the K-frame you'll be needing to change. If it's a long term project, then go for the V8, but if that's not the case, then maybe keep the 6 in it & save the 340 for another project later on. Maybe one of the cheapest things to do & easier than accumulating parts one by one is to snag a V8 A-Body parts car. Maybe a 318 car that doesn't run but is complete.
Off the top of my head, the big things you will need to change are:
Engine compartment wiring harness
Motor mounts
Fuel lines & hoses
Radiator & shroud
Engine accessory brackets
Transmission
Linkages for Trans & carb
Drive shaft
Exhaust system