Anyone done Under Dash A/C ?

With factory a/c the under dash part was only a duct with chromed pot metal vents. The vents changed to all black plastic and the shape of the duct changed a little too. Majority of changes occurred with 73 model so I'll assume these changes did also. We can interchange the vents but not the duct. They don't marry up to the plenum the same.
Oh yeah, the 68 model got lower dash pads so the put a little pad around the bottom of this vent/duct too. With or without doesn't change fitment.