68 Dodge Dart

So i have been doing the body work off and on, getting boring though, so me and my dad decided to take up work under the hood. I want to get as much hp out of the 318 in it as possible, without spending more than its worth (I realize the 318 isn't the fastest engine, and i will probably be better off buying a larger block instead of trying to add things to it like Mr. Potato Head). I also have a question about the rear axle. I'm looking to drop it and paint so it can get all the dirt off of it, but i was wondering if im better off replacing it. I'm not sure what the benefits of having a larger axle is, but is there any recommendations for a larger size axle? I have been browsing the forum about these things, glad to have people here that know what to do.