Bolt on without mods?
If you have not changed out the cam, intake & carb, and probably ignition, to higher performance pieces you will see little gain from shorty headers, except more noise and probably leaks. Stick with the cast iron manifolds and get a better exhaust system all the way back.
TTI is excellent, but overkill on a 318 without other mods. If you were going thru the effort and expense to upgrade the heads, cam, intake, exhaust, etc.. you should bag your stock engine and build a 360. They are dirt cheap and will bolt in place of the 318 , and look the same (except left motor mount); my 2 cents.