It has always been told to me that an engine is basically an air pump. That thought in mind, the more you can get your motor to suck and blow the more power you will achieve. My first mods to a 318 would be exhaust (headers and duals), secondly I would up grade the intake, carb and air filter. These are quick and easy mod to make.
Remember when you start making more horsepower the tranny will become a weak link. I had a 318 in my 69 B'cuda and went through trannies on a monthly basis (guess I should have just splurged on the high performance after market model).

My two cents,
Andy