Re: 25/35 impeller housing know-how

[Don]

Well, the exhaust manifold is the first stop on the cooling system powerhead circuit, so problems "down the line" in the powerhead wouldn’t really affect water flow into the exhaust cover/cooling elbow. So, if there is no flow to the exhaust cover/cooling telltale elbow, then the problem is before this area. So, that would leave the exhaust tube/grommet/water tube guide/water tube/water pump as suspect.
I suppose it is possible that a blown head gasket/exhaust cover gasket could create enough back pressure to stop water flow but not likely. And, leaky gaskets would probably create an internal water leak/poor running as well.
Post some pictures of the problem threads on the block, perhaps someone will have a creative repair that will save this powerhead.
If the powerhead is salvageable, then I would recommend pulling it off and going through the entire cooling system to make a reliable runner out of this engine, unless you fear corroded fasteners might create a nightmare, or the owner is unwilling to make the investment in the engine.