The 400 is considerd a big block though and with the taller decks there maybe some clearance issue's with the 63 if you still have shock towers.
You will probably have the same issue's with the 351W as well though. Also if you go with the 400 you'll have to go with a big block trany or adapter plate
to run a small block trany. Rick Devitto is running an adapter plate with a GM trany and has had very good luck with it, behind his 408W.
The 351 Clevelands and the Windsor motor's are all small blocks, If you get into the 351M or 400M those are big blocks and have taller decks on them.
Kurt