1988 Ford 351w FWC Thermostat Housing Cracked!

I'm looking for help on my 1988 Ski Nautique 2001. I bought her from an area where she ran in brackish water. This is a fresh water cooled engine with a PCM heat exchanger .Needless to say the engine was pretty rusty. Over the winter I pulled the engine (a Ford 351W) and did a paint job on it. I didn't tear it down completely as it had low hours (600). I replaced a lot of bolt on parts, fuel pump, distributor, coil, etc. I had the exhaust manifolds cleaned while they were off. 

I'm just now adding all the fluids and found a couple of things.

1) The lower thermostat housing is leaking antifreeze from a small crack at the lower bottom right bolt flange. The crack does not appear to go all the way inside the housing, just on the exterior. I suppose it is weeping through the fatigued metal to the crack? Can this be welded? Looking through previous posts it appears these units are no longer available! Part number stamped on the outside is R-025004.

2) When I opened the thermostat housing to inspect said crack low and behold...there was no thermostat installed! I'm assuming skidim.com Thermostat 170 Degree W/Gaskets PCM RP026003 is what I need? 

3) I live on Lake Ponderay in northern Idaho. Boat will only see fresh water from now on. Is it possible to convert to raw water cooled or is that a bad idea? Is there a kit available? I can't find one.

4) I used the cork valve cover gaskets recommended by skidim.com that have the metal spacers at each bolt hole. I did not use RTV on them when installing. As I poured fresh oil in the gasket began leaking oil just below the filler port! Should I have used RTV or another gasket sealer when I installed them? Is another type gasket better?

Thanks in advance for any help! 

I think you'll find that this one is cast iron 

Raw water cooled upper and lower housings are cast iron though on a 351 PCM except that gawdawful expensive replacement upper that SkiDim sells these days.

I know that, go back and read the part where I said I was wrong 

Just mentioning that the raw water cooled ones are some of that heavy magnetic metal, since it was mentioned that  marine ones were made of something that couldn't be welded.

How's your oil leak?