I assume you meant the 1 1/8" nut on the "Prop", not the coupling. Coupling's normally have little nuts, Props have the big nuts.
Put a heavy rag on the prop blade, hold it with your hand, and jam it to your shoulder. Then use an air impact wrench. You won't have to hold it with anything else.
An air wrench is much better than an electric
impact wrench, which is itself many times better than a hand wrench.
There would be no problems turning the engine in reverse, but you shouldn't subject your transmission
gears to the vibrations of an impact wrench.
Put the tranny in neutral, and hit the nut with the wrench. Use a 1/2", or larger, impact wrench, not some puny 3/8 wrench. It'll come off so fast, you'll wonder what the fuss was about.
Cheers, and happy wrenching.
PS, use a proper 6 point impact socket. Bronze nuts won't take well to using an impact wrench with a 12 pointer.
You're also going to need a prop puller, and depending on the coupling configuration, you'll need a puller for that too.
And for heaven's sake, don't listen to CaptnMike's suggestion. That's really not safe.
Use any advice at your own risk.
If you're V-drive has a coupling I've never seen before, with a large nut as you describe. Put a pipe wrench on the coupling and use the air wrench as described above.