Circled in red. The nut goes behind the lower booster brace. Its a large hex nut (about 3/4") with small threads (5/16 I think). It gives the brace more surface are so the sheet metal does not crack. Without these two braces that booster really moves around.
It goes just to the left of the oval hole (used for manual trans linkage, plugged for auto)
Pic is of my unmolested 73 340 dart.
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