A marine valve is straight on the one side and tapered on the other. Learnt this today.
A seacock is straight on one side and tapered on the other, and it has a flange that is attached to the hull. A marine grade ball valve is tapered. Groco makes an adapter that corrects this problem.
Good news is it looks like you can save the thru hulls if they’re reasonable quality. Cut them down a bit, get adapters, valves and barbed fittings. Still not cheap and will be a couple grand.