Water in oil - 340 with 8.1L

I think you're right
 
Small block; yes, but I don't think the big blocks have the same coolant passages in the manifold.

Sure they do at both ends of the heads.

Especially on aluminum heads and intakes.....retorquing is a must. If you don't the coolant passages will leak into the lifter galley or find its way into a nearby intake port.

upload_2023-4-9_10-34-54.jpeg
 
I am grateful for the comment from @Scott215 regarding the intake manifold.
Went down to the boat this weekend, and sure enough the middle two bolts on the intake manifold were not even finger tight. The four on the ends were tight, but the other 6 were varying levels of not tight enough, especially the middle two. That was one of the last things we did when when we replaced the head gaskets, and I remember running through the torque sequence. So either they loosened, or middle group of bolts was not torqued correctly.

So I re-torqued all the intake bolts in the proper sequence, and headed out for a nice cruise on the bay.
Prior to overheat, the oil pressure has been pretty close the same between the two engines. Then, after the overheat and after replacing the head gasket, the oil pressure was about 4 psi different for the port and stbd engines. Now, they are are within 1 psi, which I consider the same. Small victory! So between addressing the internally collapsed exhaust hose and the loose intake manifold bolts, we are making headway.

The boat ran great. Both engines were smooth and solid. All the gauge readings are back to what they had been before the overheat. Now I have a good 1.5 hrs of run time on this new oil. I have 3.5 hrs before the next sample goes back to Blackstone.
 

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