If you want the highest gas miileage you can get, along w/good performance & driveability, you pretty much have to go w/fuel injection. Look at the GM crate motors that are already set up w/FI or buy used. I would not pick a 305 but a 350 with FI & a overdrive trans.

If you want to buy used look at camaros,trans ams or vette.
good luck w/your project