Jer - If you're referring to the nut I welded inside the driveshaft tunnel (green TR3A) I welded each of the nuts to pieces of 14 gauge sheetmetal about 3 inches square. Just before welding the nut into place, I hammered the sheetmetal plates into a curve to fit the contour of the curve inside the tunnel. Then I welded on the nuts, then I tacked the curved plate into place behind the hole you see. There is one just on the other side for the other seatbelt. A tack at each corner is good enough because it's only to hold it all central with the hole you see because the bolt will hold it all together in tension.
The nuts I used were stainless nyloc nuts. The welding warmed up the nyloc a bit but the bolts holding the seatbelts have never come loose on my black 1958 TR3A in 18 years and 100,000 miles.
Look over to the far side where you can see the interior of the quarter panel just aft of the "B" post, where you can see where I welded another nut to the far side of a piece of 1-1/4" inch steel angle shown vertically which I welded down on the outside of the outer sill before I welded on the quarter panel.