Funny you should ask - I just pulled some this week from a Flint built '70 Skylark. I'll get a pic. Also pulled the under dash A/C plenum. Still not sure what part you need.
That piece of vent is gone from the car I just pulled apart but one of the others should have it. Here's the screws I just pulled from the driver's side kick panel on the '70 Flint AC Skylark.
Buy a Chevelle seal kit, probably from Groundup. Those are the screws, but I think offhand there are 3 of them (pass side) and one is longer.