Unless you have one of the really rare late 1971 blocks that where cast for the 400 motors you need to be very careful on tuning, no detonation Most of the 383 blocks had thin main webs like most RB blocks do If you have a forged steel BB crank I would have it offset ground down to BB Chevy rod journal size and the stroke to 3.91 or 3.90 stroke and have the mains and counterweights cutdown to use in your 383 block, those motors flat rock I've built a bunch of Mopar V8 of all sizes and types, the 3.91 stroke B and RB motors work really well on the street and strip