Your heater resto looks great - I'm starting to convert an 89 City to Monte Carlo replica Mk1 - the main body of my heater looks almost identical so I'll just adapt it with a new front panel and a lever welded to the flap.
In pictures I can see what looks like little downwards facing teeth in the slot on the Mk1 front panel - can you tell me how the flap lever is tensioned upwards - is there a spring somewhere?
They are downwards facing teeth like this: _\ and they are on the left hand side of the slot. The lever on the flap is then bent slightly to the left hand side so when fitted through the slot it is naturally is tensioned towards the LHS of the slot and sits under the teeth that stick out. When you want to change its position you push it over to the right and move it up or down and it will hold itself under the next tooth.
Hope that makes sense,