A couple of things I might suggest.
First the engine. The 305 is an okay engine but it doesn't respond as well as some of the other popular SB's to performance modifications due to bore/stroke ratio and head limitations.
If it is at all an option, I would propose you discuss building a 350 to swap into the car in place of the stock engine with you parents so you do not render the stock motor unuseable if you modify it. They might be pleased to let you work on the replacement engine while you learn about the car and the mechanics necessary to build and maintain a performance engine and vehicle. The 350 will cost the same amount, if not less, to build as a 305 and will give you a much better HP per Dollar return for your investment.
If you do decide to try some minor Hop-up modifications to the 305, such as headers and intake, leave the "Cat." alone - it does not restrict the exhaust any great amount and removing it effects the emmisions system of the vehicle for inspections.
Good luck with your project - you will find all kinds of good info on this site!
[This message has been edited by Vintage 68 (edited 05-08-2002).]