You might try Port Harbor Marine, but that's Maine. Closest location is in Kittery. It is a haul, but you should only do an install once. I can ask a guy who used to work there what he thinks. I respect his opinion.
I think the best price for the parts alone might be the place in Worcester. But the installs are not great. Mine had to be re-done. Pull the engine, etc. And the glass work was nothing to write home about.
I think it was IslandTrader who did it right- he got raw water cooling in a 383 (
http://www.classicseacraft.com/commu...=18594&page=14 ). You will need expensive risers if you use heat exchanger/ fresh water cooled exhaust. And many (hundred$ of) manifold parts need still need replacement every 5 years or so. I am pretty sure I would go that way (raw water), if I did the install. I think. I haven't found a great alternative.
Or you could go big block with injection:
http://boston.craigslist.org/sob/boa/4176765292.html