I believe the nut is Ford hardware part number 376903-S40, a 1/4-20 thread hex body (for retention).
A follow on -
The "original" nut did have a hex body retention method. The replacement nut keeps the same Ford hardware part number and Ford Service Locator Code of "MM-120-BF", but is shown in the 1969 Ford Standard and Utility Parts Catalog (a Jim Osborn publication, AM0205) as having a round body, as if it uses a crimp type retaining method, like a pop rivet does. Several years back, I bought a "pop-rivet" type tool that is capable of crimping in "nuts". (I don't remember where I bought it.). If you are not in the thoroughbred class, this tool and 1/4-20 nut "may" work, depending on the original size of the hole.
Jim