Shark Racer

So, I'm converting my 78 over to a dual snorkel setup and I have everything except the fasteners and speed nuts for the passenger side air duct. Does anyone know the screw size? Looks like I'll need a speed nut that has a minimum of 5/8" from the end of the clip to the centerline of the "nut" circle.

I'm able to find 1/4-20 locally but that's as large as the diameter of the hole in the plastic duct itself.

Anyone happen to know or would be willing to check? Close up photos of the screw heads would be nice if possible...

Thanks!