Instead of rebuilding that 305 you can have a chevy 350 crate motor put in for cheaper probably and have more power. This is my suggestion... Any idea on what rear gears, tranny you have? We need more info here to help you out. But if you absolutely have to keep that 305 and don't want to rebuild it yet then first thing is headers and dual exhaust. After that and intake manifold and Edelbrock or Holley 600CFM carb would do... Unless it's TBI.
That right there is a start. Not too much you can do to a stock 305 with all those miles on it...