I'd look at your alternator as a cause for dim headlights. Check the main lead coming off the back of the alternator and going to the positive side of the battery. Also check your engine to body grounds (where ever they are).
As far as extra power goes...
http://www.500cid.com/
You know a 425, 472 and 500 are all the same block externally right? You could change the engine and gain 75 cubes and still have essentailly the same engine externally. There are all kinds of changes you can do and most involve changing the cam out. You can also put headers on it. 425's aren't bad, but why not have the extra 75 cubes for almost the same weight?