If memory serves me correct, the flaps are controlled by a slide switch mounted on the accelerator pedal. But The engine has to be at a certain operating temp (normal). The shaft that the pedal hits to activate the switch is about 1/2" round. I put a piece of pvc on it and made it bigger to open it sooner. There are probably several ways of doing this, this is just the way I did it about 18 years ago.
1969 Sport Coupe w/97 LT1 - T56