Should I go Big block or Small block?

318 stroked to a 390 or a 360 punched out to a 408. Can't go wrong with a basic 360 either. Small blocks are alot less headache in a Duster/Dart than a big block, especially when it comes to exhaust.

If you just had to go bb, then forgo the 440 and do a 451 low-deck. Lighter weight and more powerful IMO.