I've worked on a few inboards for friends and neighbors, so I know just enough to be dangerous. I also am on my 2nd inboard/outboard that has a 4.3 liter Chevy V6 built up by Mercury for their MerCruiser system. There is a lot more to an inboard engine than just hooking up a car engine, especially where safety in involved. If the motor is going to be in an enclosed space, you will need water-cooled exhaust manifolds and spark arresting air cleaner (the air cleaner is law in most states and with the coastguard). You will need rubber fuel line approved for marine use. Check out how cooling water gets to these inboards too. Most have an impeller and an automotive type water pump. You also need spark-arresting screens on the alternator vents. Need a blower to purge air from the motor enclosure for a couple minutes before starting in case gas vapor has accumulated. Most inboards and inboard/outboards use a 150 degree thermostat. Best if you find a junk inboard boat to donate the drive system, prop, steering with rudder, and other stuff, some of which is mentioned here. If the motor is not covered up, and you are looking at straight out exhaust pipes used in racing, etc., be advised they are illegal due to the sound level.
A lot of today's name brand inboard boats use engines by Pleasure Craft http://www.pleasurecraft.com
They build auto engines up for marine use. My neighbor's Ski-Nautique from the 1970s has a Pleasurecraft Ford V8.
Back in the 1950s/1960s, magazines like Popular Mechanics had plans to build many types of boats. These and other plans are available at Glen-L and Clarkcraft, and probably others too. Many of these were inboards. I recommend you look into some published plans and see what all goes into an "old School" inboard. There are kits available too from some of these companies:http://www.glen-l.com/designs/inboard/inboards.htmlhttp://www.clarkcraft.com
An old school inboard canbe a very cool, fun boat if done right!