Thanks guys for your replies. I'm not sure I followed all your advice since I have so little experience with seacocks.

I did know the "right" thing was to change it out to a proper seacock. When I think about how much work I'm putting into this boat already it seems kind of silly to take a shortcut on something as important as this. It would be bad enough if the boat sunk with nobody on it. But even worse if my family was on it.

So now maybe you can help me understand the things I don't understand so I can do this right.

With a conventional Bronze seacock you have a thru hull mushroom fitting that goes thru the hull and screws up into the seacock. I do not seem to find any similar fittings (the thru-hull part) to go with the Marelon seacocks. So how is that handled?

With Marelon seacocks you still bolt thru the hull. Are these bolts Bronze or stainless or???? And do you typically use a carriage bolt or a countersunk head with screwdriver slot or?????

When it comes to the blocking inside the hull can I simply use some of the 3/8 thick fiberglass panels I have here and set the block in a bed of thickened epoxy to get the flat surface for the seacock to sit on? Or is there a better way?

It sounds like once I get the old glass tube out and the hull patched in that area it might be a good idea to move the sink over to the port side so I have a fresh area of hull to install the seacock in. I'm completely redoing the inside anyway and this would not be hard for me to do.