I started out with a stock '79 boat full of emissions. The "before" picture is below.

My most anticipated performance add-on was Mark Allisons' headers and a new exhaust system. His headers are the coolest headers I have ever seen. And posts from the pro's say that they perform well too. So Mark's headers were a no brainer. I also added a rebuilt 1800 head (from IAP), a single plane manifold (from Mark) and a 32 ADFA carb (also from Mark). I learned a lot from Brad Artigue's guide to "Intake, Combustion, and Exhaust Modifications" posted on mirafiori.com.
Below are a couple of "after" pictures and a close-up of Mark's headers.


From a performance standpoint, I do not have any dyno information. But I can tell you that it has a noticeable increase in response. I feel much more "connected" to the engine and drive train and it breathes so much easier. I am very happy with the increased performance.