I was able to get a good look at all the ammeter wiring from under the dash. The splice looks good...
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...and the connectors on the back look original and clean. Could not verify tightness because I could not reach them (only see them), but no reason to suspect they are loose.
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Back to the ignition switch wiring, I found a not-so-great splice into it for the aftermarket tach, and a bunch of loose electrical tape instead of shrink tube and cable ties, so I tidied all of that up.
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Still plan to swap out the switch. I'll do some searching for a suitable fusible link to install at the starter relay. Looking forward to the info on the one you used. Thanks for all your time, suggestions and advice. It is much appreciated!