you are very right, Gustavson heat exchanger is quite different from Lehman’s stuff that – according to David – is easy to remove and inspect.
I did it 15 years ago on a volvo 2040. very esay task.
Thus I tried on the Ford Gustavson and never came to an end as I became too frightened to break something.
I started with the 3 bolts on the starboard side of the fresh water tank. This is the place where both seawater and engine oil pipes enter the heat exchanger. The 3 bolts themselves were easy to remove and then everything went too dificult for me and too dangerous with that old engine and its hard to find spares.I never succeded removeing the cover held in place by the three bolts.
For your information I vaguely remember I passed along to Richard a diagram of Gustavson water tank I got from someone on Nauticat USA forum. Very helpfull.