what does it take to swap a big block in a duster ?

With the limited space afforded by a 440 I would opt for the B 383-400 set up especially I you are going to install a turbo. Depending on the car the clearance between the shock tower can be at an absolute minimum. One car will accomodate a 440, the next one requires chiseling and frigging to make it fit. I am assuming this based on my thinking that you will fab or have fabbed headers to drive the turbo.