How much rust are expecting in these motors? Seems like a lot of work for just a little but of surface rust. Most fresh water cooled BBC's, even in salt water, are not completely rust clogged unless they were abused (ie never fresh water flushed).

Yep, I'm planning on doing that during the final flush to encourage any additional bits to flush out.

Blocks have only seen brief periods of salt water usage with prev owner and they were flushed afterwards. However, the blocks are 35 years old.
On a separate thread I had mentioned that this season, when fired up, I got a lot of rusty water out of the starboard eng. Enough to stain the gelcoat, SS prop and trailer bunk carpet. Smelled bad. Engs had been stored for 3 years with RV anti-freeze which I thought had some anti-corrosion properties, but didn't work very well.

Once I get them cleaned up as best I can, I'll be using green anti-freeze to winterize from this point going fwd.

Got the treatment (Thermocure) into the eng and up to 160 degs before shutting off. Went pretty smooth except It did burp on me and made a small mess.

For the exhaust cooling hoses, plugged the hoses for the exhaust elbows. Used a garden hose Y adapter for exh manifold water hoses. The hoses fit right over the garden hose Y adapter threads and used the orig hose clamps to secured them.

Plan is to start the eng a few more times this week and drain/flush next weekend.

Got the rust treatment flushed out. It was definitely dark brown but not black like some online videos showed. Looking thru the hole on the intake for the thermostat housing, I can see there's a little bit of rust still in there but does look like it took most of it out.

With the thermostat housing, I coated it with Ospho inside and out. All the surface rust turned black and if you wipe it with a rag, nothing comes out on the rag. I'll see what it looks like next season.

Didn't put Ospho into the block. Wasn't able to find much info abt that. One commenter claimed it would block heat transfer to the water??