The Vortec heads would definitely help with the power, but I would be curious how the MPG would be between the Vortecs and the stock heads. The Vortecs should be as good if not better for MPG since they have a more efficient combustion chamber. Plus in theory an engine with more power requires less throttle opening to create enough power to move the vehicle.
Have you read this article on a 307 build? It's not what I'd call a budget build, but it may give you some ideas.
http://www.superchevy.com/technical/...nce/index.html