Cleaning water jackets 225

No don't worry about the alternator; the limiter buffers most of any voltage change there. And as for the fuel gauge, it is not all that accurate even when right! So maybe I should say 'reasonably functional', rather than 'accurate'. If it read 1/4 to 1/2 tank when it was actually almost empty, or was at or near full most of the time, then that would indicate the voltage limiter was too high. Since you fuel gauge responds

The limiter itself is hard to test. All you can do is take the wire off of the temp sensor and read the voltage at the end of the wire with the key in RUN. The voltage will pop up and down at rate of about 1 second per cycle; it is hard to see this on some digital voltmeters. This voltage changing indicates the limiter is working, but does not determine its accuracy.

But at this point, I would be changing the temp gauge based on all the symptoms you have presented. BTW, if you take it apart, you might find some tiny adjustments; see here:
http://www.netlink.net/mp/volks/htm/fuel_ga.htm