My understanding, on Galaxies anyway, is that the cars came from the factory with undrilled fenders. The dealer prep included drilling the antenna hole and installing the antenna.
So, based on that info, I'd say you won't find a plug. Rather than welding a plug, just get a stainless steel hole plug (may have to enlarge the hole depending on the size you find) and install that. Not sure what to tell you about the screw holes. Awhole stainless or chrome or aluminum plate over the antenna area may be what you need...
When I purchased my '64, there was a piece of metal attached to
the underside of the fender where the attenna was supposed to
be but no hole whatsoever. I gather that this is the way they
came from the factory or else this was a stiffener plate. Anyway,
putting a plug in the hole will look just plane trashy so I would
recommend removing the fender and soldering a piece under-
neath and then lead filling. Painting the fender to match should
not be a great expense. Barring that, mount a dummy antenna
which would look more asthetic than a plug.
FWIW, I removed the antenna from my Stang after it got painted as well. I took a square piece of .040" aluminum, shape it sort of an elongated hexagon shape, polished it, engraved the "pony" on it, clear coated it and stuck her on top. Looked REALLY good!