v-6 in early A-body

Economy....you wont get 25-27 mpg hwy with a 5.9. Cost is another issue as 400+ HP will require 4 wheel disks, larger rear diff,beefed up trans, larger radiator, electric fan etc.
SSVDP what mpg did your Dak get on frwy, mid 20's?
Considering a 3.9 is about 100 hp and 100 ft lb torque over a /6, thats a huge improvement for the price of /6 header, cam and intake/carb and you end up with injection for daily driveability , which a 5.9 would also have.