450 to 550 is a big step for a 383, I would go with your #250's for 450hp and eddy rpm's for 550hp.
Check out the mopar muscle web site they did a 450hp 383, it was very basic unported heads, 10.2 comp, 557 cam, mi and headers, enjoy
Good point. The OE heads can do OK on the street and the 450 HP target isn't hard to get or do. Even with the OE valves, basic porting and back cutting will get ya there for less than the new aluminum heads.