help identifying A/C part 1972 k20

[nitrosport_5]

I am adding factory ac to a non ac cab. I'm to the point of installing the fresh air flapper in the cowl. I have the plastic bushing piece for the engine aide, but what is used on the glove box side? I know there is a pre-marked spot to drill for it, but what sort of bushing goes in there? And what size hole? Looking at my 72/c30 ramp truck it looks like a nut, but if you try and screw it out it is never ending. Any help is greatly appreciated. I pictured the flapper I was talking about.

Thanks ks