Sb/bb/...bb???

I'd drive the lug nuts off the Duster till you have the BB built and all the bugs worked out. You can beat on the Duster during the time your acquiring all the needed parts. Then, once you have it together and all the problems fixed, (Yes, there will be some) you can swap the drive lines from both cars. That way you have a minimum amount of down time and still get to enjoy driving the Duster chasing parts for the bb buildup. Then after you have them swapped and the big block up and running/driving, you can tear the 340 down and build up another Stroker for the Dart.
You can even keep an eye out for good deals on parts for both cars and snatch them up as you get the money, but main focus on getting 1 or the other done, then concentrate on the other driveline.