Looking to replace my 273 with a 318, which one?

If your 273 is a stock 2bbl I kinda understand, but a 318? If you can find one go for a 340? That said, if your 273 is the 4bbl "Commando" version restore it! This is a kick-*** engine and will get good MPG with 3.23 rear. Put a 4spd on it and you'll have a ball with it.