What is your compression ratio? Are you wanting to replace components? What kind of exhaust system are you running? Headers? The carb seems a little big for stock heads. Have you thought about aluminum heads? How about a nice stroker kit? Try these guys. http://www.440source.com/ These are a lot of questions but how far are you willing to go and what is the purpose of the vehicle? Street or strip or a combo? You should be able to easily get a streetable 400 hp but it is going to take some component swapping. The 383 is a very good engine but there is no replacement for displacement.
Bob