I have an LS 6.2-480 engine that runs well. I’ve marinized it using aluminum exhaust manifolds from a Kodiak... but I kept the original valves. I’m not worried about its operation during the summer, but I’m not sure what to expect during winter storage.
The engine has been flushed with fresh water, but I haven’t added any kind of preservation or corrosion inhibitor.

I’d really appreciate some advice on what (if anything) I should do to prevent damage or stuck valves—or any other issues.
The boat is stored in a garage from September until June.

To an engine stored in a warm garage?

Run on fresh water to clean out the salt?

the simple answer is nothing

if you want to complicate things a bit, run it with Chevron Techron Marine (PEA+stabilizer), then pour a little storage oil down the spark plug holes and turn the engine over by hand

I would just leave it

Valves getting seized/stuck are a rare event. LS factory heads typically have powdered iron guides, valves would have chromed stems. (You need chromed stems to work in iron guides. Stainless stems in iron guides increases guide wear.)

Raw water cooled...do a fresh water flush. You can fill the system with an anti-freeze coolant to arrest any corrossion.

Thank you!
Antifreeze it is in.

Boat it is stored in the dry, but not warm garage.

Dry is what matters

If it's going to be below freezing, make sure that all the water is drained from the block, oil heat exchanger, water pump, exhaust manifolds, etc. or filled with antifreeze.