What you are wanting to do has been done many many times prior.
No real issue with an HF7 to my knowledge. Done it myself and it's a strong gear.
There are differences in the block of the original 283 and a 350 core most notably the crank case breather. You will have to vent the block through the value covers. You will also have to think about carb/cam/compression selection as well. Also keep in mind that the flywheel forward CC is in the boat backwards in relation to the car application. So things need to be set up to turn the right direction.
PS. You can certainly get more than 250 HP out of the 350 SBC. I would expect a base of at least 270 but not that hard to get 300 HP.
Other things are that your engine box will be way bigger than you need so you might consider making the smaller one.
My first question is why not go back the the big block Lincoln? Keeps boat more original, better value, Hell of a torque monster engine...
I have owned three of these boats and rode in other big block versions and can tell you they are a much different experience.
Also that boat was delivered with the Chrysler 413 which is a Hell of a motor. I'd also consider putting in one of those and keep the boat a big block.
PS. Big block boat also had the exhaust in out locations.
You see there are a lot of things you need to consider that are going to not only take some cool out of the boat but IMHO greatly de-value it in the future.