Fender bolt

[fireguyfire]

I’m just reassembling my 1970 D100 after restoration, and I’m having a brain fart on a fastener.
There is a square hole in the front fenders, halfway down the inner rear of the front fenders where a bolt goes through into the inside of the cab to adjust the in/out of the middle of the fender in relation to the front of the door.
Whatever bolts were there are gone, and I’m wondering what type of fasteners go in these square holes?
Are they carriage bolts with large washers on them?
Any help would be appreciated!

[PwrWgnDrvr]

The square hole needs a 5/16" speed nut that clips into the hole. The bolt comes from inside the cab and threads into the fender. it is NOT adjustable.

[fireguyfire]

Thanks Powerwagon; is there a way I can rig up something similar?
Mine are gone.

[PwrWgnDrvr]

I've welded a nut into the hole. I figure that's better cuz it will never break loose and spin when trying to get the fender off. That's an ugly hassle!

[Series1Utiline]

I used these from Fastenal:

5/16"-18" (.028-.056 Panel Range) Plain Steel Cage Nut. Fastenal Part No. (SKU) 37974

[Series1Utiline]

I used these from Fastenal on similar holes on the transmission tunnel. They have t hem in various sizes:

5/16"-18" (.028-.056 Panel Range) Plain Steel Cage Nut

Fastenal Part No. (SKU) 37974