hotshoe,
When was the last time you inspected and adjusted your fan belt tension?
Are you using the correct fan belt size to match the 216 engine pulley? [modern fan belts sit too low in the root of the pulley]. Many fellows use the wrong size fan belt , just like the wrong size of battery cables for 6v trucks.
The correct size of fan belt rides just a hair thickness above the pulley.
The fan belt I am using is made by Dayton, model 22435.
You will generally see a slight "discharge" reading in your ammeter at idle.... you will also see a slight "charge" reading upon acceleration.
6v systems do not charge your battery at idle...unless you have your RPM's set way too high.
I do not profess to claim that what I write is the correct solution...diagnosing electrical gremlins over the internet is a crap shoot.
I hope this electrical glitch you observed does not stop you from attending our meet & greet on February 10th!... remember, it is all about meeting a fellow bolter....we can always see your truck in the Gallery!
Greg