Not a Chevelle, but my 64 C10, stock 350, Holley carb, manual transmission...

Car runs absolutely normally under load. When you come to a stop and put the car in neutral, it will idle totally normally for 60 seconds or more, then the idle starts to drop and the engine eventually stalls. It will crank back up after many turns of the starter, but then run completely normally again, at least until you stop at another traffic light. Then the same thing...idles normally for a good little while, then peters out and dies.

This started after I poured 5 gallons of fuel into the tank from a plastic fuel jug. Did I possibly get a piece of trash or sediment into one of the idle orifices from the fuel jug? Or is this a fuel pump issue? Clogged fuel filter? More and more I'm thinking the fuel pump isn't pushing enough fuel at idle, but I welcome any suggestions.

It sounds like its slowly loading up . I think you may have a stuck (float)needle. Maybe some trash is clogging the needle therefore never shutting down the fuel flow to your bowls ,thus causing a slight drip into the manifold and causing load up .

if this is a holley carb check the needle/seats.

That's a good thought, and easy enough to check I guess. I'll pull the needle/seat assemblies.

Good call Jason, that's what it was...trash in the needle & seat. Looked like black paper gasket material.

Truck has a fairly new fuel tank sending unit with new sock, and an Earl's inline fuel filter between pump & carb. I have no idea how this stuff could get into my carb.