Summit or Jegs 2.5 kit works great and is reasonably priced and mandrel bent. Used the kit on both my cars. Comes in a white box but is made by Heartthrob Exhaust. You would have to fab the down pipes.
Then again..... If a 340 or 360 is in the future, build the exhaust for it now and worry not about the current engine
If the 318 is very close to stock, a 2-1/4 pipe works excellent in a dual exhaust system. Use with a decent flowing muffler and it will be fine. But on a 340+ size mill, of any type of performance level beyond stock, the 2-1/2 pipe is what would really be a benefit.
