Halekai- Since you are the local expert, can you thread the seacock on over the thruhull nut? When I threaded the ball valve onto the thru hull it threaded on pretty well, but was "stiff". (now I know why) Good thing I'm not a plummer.
I'm not sure I have room for the flanged seacock (near motor mount and not much clearance.), if not is there a thread adapter to get to the type of thread on the ball valve.
FYI, it's almost under the engine with exactly zero chance of anything ever rolling, sliding, falling, dropping, or otherwise moving into / onto it.