Overheating due to clogged v-drive fitting

[Trees]

Greetings,
After hassling for months with an ever-increasing overheating problem, it looks like I finally found it. I searched through these forums and the web and didn't find a reference to my problem (of course, I probably just missed it somewhere), so I thought I'd post it for possible reference for others.
I had steam starting to form and increase while the coolant outflow slowly decreased. Acid flushing, pressure back flushing had little if any effect, although I had good flow from the thru-hull and out of my raw water strainer.
Turns out that the 90 degree fitting coming out of the v-drive coolant tube was filled with what appears to be impure flattish salt crystals, to the point of almost a complete stoppage. There was no blockage at all in the coolant tube itself or in the input fitting.
In defense of myself, and why it took me so long to figure this out, I'll point out that, besides being a rookie, my boat is 400 miles from me and I get there, if I'm lucky, for several days, once a month.

Trees
1972 Columbia 30
Poco a Poco