Sure feels like a head gasket to me as well. Here's my thread on this very subject. Different engines but same symptoms.
I had removed the head 3 years prior to this due to a bad valve and so had put on a new head gasket at that time. Seemed implausible that it would go bad so soon. I tried throwing parts
like thermostats and radiator caps at it hoping it was maybe a simple overheat issue.
In the end it's not that big of a deal and really not that expensive assuming you would do the heavy lifting. The combustion gas analyzer is the best way to check but since you already have exhaust gases coming out of manifold you can save yourself the $60 and just assume you have a bad HG.
Your comments about sludge getting on HX and overheat aren't the same experience as mine. My engine
would also run hot and here's what I found. I had a leak between cylinder 2&3 on my Westerbeke
. The compression
of the engine is 450psi. Which is enough to force combustion gases past the HG and into exhaust manifold. Once there, and given time, it would force the coolant out past the radiator cap and up to the expansion tank. Once enough coolant has been forced up to expansion tank, there wasn't enough to circulate around engine and the temp would rise.
What made it even worse was slowing the engine down. Every time I would have an overheat alarm
it was when I throttled down, presumably from lack of circulating power at lower rpms.
Anyways I think the fix for you is to find a good machine shop that can inspect the head. They do this either with a precision straight edge and feeler gauges or with a surface plate and Dykem. I have both here in my home machine shop so I checked it myself and also had machine shop check it. If you look at my thread I posted a pic of the Dykem (ink) and the blue spots were where the head was low. I also had the machine shop magnaflux the head which is a way to use special powder and an electromagnet to check for cracks in the head. That came up negative and the machinist said it wasn't necessary but I wanted it done belt and suspenders.
So other than my time, and I had all the tools like a torque wrench, it was $75 for a new head gasket, $90? for the head to be machined with CBN tooling, $180 for magnaflux (skip this) and some odds and ends.
I think hardest part of the job was preparing the engine gasket surface and removing all old gasket with a razor blade and not getting crap down into engine. I used a variety of expandable rubber plugs, shaving cream, tape etc to keep it as clean as possible.