'63 Valiant V200 wagon - rare bolt issue

jeff alder said
are you using a tapered 1/4 20 tap or a bottoming tap?

It's tapered.

I'm not going to fuss and cuss trying to tap it from the inside; if I can't get the bolt, I'll do on the passenger side what I did on the driver's side, which is drill through the tailgate to tap the hole.