lightest A body

1963-1966 Valiant 2 door post car would probably be lightest. Keep in mind those cars don't come without hassles though, namely tire clearance and narrow engine bays.

'67-'69 Dart/Valiant post cars are pretty light too. They have a little more tire clearance and a standard width engine bay.

Dusters are probably the lightest of the '70s A bodies. My all steel '71 with full interior, frame connectors and iron-headed small block weighed 3,060 without me and a full tank of gas. I shaved some weight but it's a street car.