Hello from Leipzig, Germany to all

Welcome to the club!
As far as your engine choice of a 340, they are not cheap. For the price to get your hands on one (and have it shipped to you), you could build that 318 or buy and build a 360 for cheaper (hell a stroker on either the 318 or the 360 will produce a lot of torque which king for the street!). That being said if you plan on screaming down the autobahn, then rpm is what you're going to want so I guess a 340 would be better for the job. Does the Duster already have an 8.75 rear end in it (these look just like a 9" ford)? If you at least have a quick gear change setup.