The most defining difference is that a DD is limited to turning about 3000 RPMs.
A gear reduction alows the engine to spin up to enough RPMs to provide usable power.
A 350 at DD RPMs (3000) is not producing very much usable power.
A 350 w/ a gear reduction (5500) will produce usable power.
This is why you see so many big blocks as DD engines.
Cubic Inches make low RPM torque, a factory small black usually does not.
In my opinion, a 14x8 foot hull with a 350 DD is going to be under powered.
That hull would be a good candidate for the 350 and a gear reduction, or
as a DD, that hull would work OK with a 500 ci Cadillac.
For a 350 DD, a smaller hull would be much more servicable, imo.
Suggest you read a lot, get some experience and look a lot before you spend any money.
No disrespect intended, but if a unscrupulous seller heard such a question,
you could very likely end up going for a very expensive first ride.
Welcome to the forum, you've come to the right place to ask questions !!
There is a [Search]
function up towards the upper right side of screen, answers to
most common questions are there. If not, somebody here knows or will find out.
There's more airboat info to be had here than everywhere else put together.