Ok , need to step back

I would sell the Duster and the 340 together as a package.
Keep the Dart as is or detune it slightly.
Gas mileage isn't a concern to me if you aren't driving distances of over 200 miles at a time.
Once you've had a big block in an A-Body, it's hard to turn back.
The torque alone will steal your heart.............
I like the idea of the 340 and as far as missing the torque, my 354 had probably more butt grabbing torque then the 470. The 340 should be very streetable.