Tennessee, No harm intended, bud! We've all been on your side the whole time! I figure he might not want to go to the trouble, I'm sure you can get a fair price for yours with a bad engine, but you've gotta find someone who wants to go through all that.
First thing I would do is a leakdown test on that weak cylinder to determine what part of the cylinder isn't sealing, i.e. rings/piston or head/valves. If it is head related you most likely be able to just pull the head and have it repaired, not have to pull the engine.
If it's more serious, you might consider buying a rebuilt long block or even a used LS400 engine. I'm not sure how much if anything is really "different" with the car engine. I know things like the marine specific parts, i.e. throttle body, exhaust manifolds would probably bolt up. I wonder if the cams are different.
Either way, I would bet dollars to doughnuts that you could swap a car engine in, even a used one, and that it would work fine. Engine rotation can be reversed in the transmission, I hear, on that boat. I don't know if the car engines rotate the opposite, just mentioning.
I don't live too far from there, I'd run up to help you pull a head or even swap an engine in. If you had a place to do it, etc. Overhead chainfall, no cherry picker. A tree works;)
My dad was a TV repairman I have the ultimate set of tools.
